Fisheries Act 2020 - not possible to enact when a member of the EU because it would have been contrary to the Common Fisheries Policy.
The Fisheries Act 2020 is the most well-known example because fisheries has long been a high profile issue in relation to EU law. Many UK fishermen lost their livelihoods as a result of the Common Fisheries Policy. Also the first time an Act of the UK Parliament was disapplied because of incompatibility with EU law involved the Merchant Shipping Act 1988 - a measure designed to prevent non-UK fishermen from using UK fishing quota by the device of operating through a UK registered company. The Act of Parliament was eventually declared ineffective in a series of court cases stretching over 11 years from 1989 to 2000.
Removal of VAT on women's sanitary products was actually "enacted" in section 126 of the 2016 Finance Act but with a proviso that it would not come into effect until the "appointed day". EU law allows members states to set rates of VAT but only within certain bands and zero-rating is only allowed for certain products which do not include women's sanitary products. Zero rating was brought into effect in UK law as soon as that became possible when EU law ceased to be directly applicable at the end of the transition period. The actual legislation bringing zero-rating into effect was regulation 3 of the Finance Act 2016, Section 126 (Appointed Day), the Taxation (Cross-border Trade) Act 2018 (Appointed Day No. 8, Transition and Saving Provisions) and the Taxation (Posttransition Period) Act 2020 (Appointed Day No. 1) (EU Exit) Regulations 2020/1642
Immigration and Social Security Co-ordination (EU Withdrawal) Act 2020 introduced the points-based immigration system restricting the immigration into the UK of people who are neither British nor Irish citizens. This would be contrary to EU Law which allows, in practice, the free movements of citizens of all EU members states into any other member state. The EU regulations in theory only permit immigration in order to take up a job in another member state but they have been progressively interpreted by the Court of Justice of the EU to include job seekers so that in practice there is complete free movement.
Abolition of Francovich actions
Francovich action were prospectively abolished by Schedule 1 of the European Union (Withdrawal) Act 2018. By "prospectively abolished" I mean that the legislation was passed in advance and became effective at the end of the implementation period almost a year ago. The abolition of Francovich actions does not get much publicity because it is complex and a bit technical. EU law is enacted in one of three ways. Some provisions of EU Treaties are directly effective and create EU law. The EU also makes regulations which also come into effect directly within member states. The third form of EU law, and the most common, is directives. A directive requires all member states to alter their law to accomplish a particular result but (unlike regulations) it leaves it to each member state to decide how to change its own internal law to achieve the result the directive requires. The idea is that each member state has a different legal system and the law will therefore need to be formulated differently in each legal system.
It is a key principle of the EU that no language has priority. Every directive is issued in every language used by a member state and every version is equally authoritative. So, for example, if some aspect of the English language version of a directive is a bit unclear, you can't say "let's look at the master version in French to see what was meant" because there is no master version. This means that the language used in directives has to be fairly general to ensure that the translations correspond - very precise language of the kind used in, for example, UK legislation - cannot be used because a precise translation would be impossible.
When the UK government received an EU directive it set about drafting UK legislation to give effect to it using the very precise language which is used in UK legislation. This process involves risk. Because of the imprecise language used in the directive there is a risk that the precise language of the UK legislation will, in some respect, not quite produce the full effect which the directive might subsequently be held by a UK court to require. So, for example, an employee might bring a claim for holiday pay against an employer. The tribunal might rule that the employee is entitled to the payment they are claiming under UK legislation. Or the tribunal might rule that they are not entitled. In the latter case the employee might leave it there but in some cases the employee might be advised that the only reason why they lost was because the UK legislation "incorrectly transposed" the EU directive into UK law. In this case the employee can bring a claim against the UK government on the basis that it is the fault of the UK government that their claim against their employer failed. This is called a Francovich action and is part of the idea of "dual vigilance" established by the CJEU. Essentially the EU incentivises individuals to bring claims for breach of EU law against their governments in addition to the Commission bringing proceedings against governments.
"Dual vigilence" was not part of the EC treaties but was developed by the CJEU:
According to this model of enforcement there are two routes... for
tackling action that is contrary to the Treaty. The first, the
"European-level" infringement procedure... was clearly marked out in
the Treaty from the day of its entry into force... The second, the
"national-level" control, was not set out in the Treaty. It is the
child of creative jurisprudence...
Cases and Materials on EU Law, Stephen Weatherill
Francovich claims tend to work their way up the hierarchy of the UK court system - Employment Appeal Tribunal - Court of Appeal and eventually to the UK Supreme Court. When they get to the UK Supreme Court that is not the end because EU law requires the Supreme Court, if it is in any doubt about the meaning of a directive to refer to matter to the CJEU. Once the CJEU has ruled on the meaning of the directive, the Supreme Court decide the case which may mean that the UK government has to pay out the holiday pay which the employee for some reason was not entitled to under the UK legislation.
But by that stage it will be many years after the claim was originally made and there will be thousands of similar cases in the wings which the UK government also has to pay out on.
This system is hugely costly and is really an inescapable part of the way EU law operates with imprecise directives (which have to be imprecise because of the language issue) opening up the most careful government to retrospective liability. To try to reduce its risk the UK government often uses gold plating. In other words it goes beyond what the directive seems to require so as to reduce the risk of Francovich actions. Gold plating can cause undesirable consequences. Where directives provide for exceptions they often have an important rationale - gold plating can reduce the scope of exceptions and impose unfair burdens on businesses which the EU legislators did not intend.
Gold plating has serious effects on political accountability. Obviously the whole concept of the EU reduces political accountability because elected representatives in the UK parliament are unable to legislate contrary to EU law and EU law is not decide by the directly elected EU parliament (which has few powers) but mainly by the EU Commission. But gold plating introduces a further problem for political accountability.
The problem is that a citizen may complain to their MP about particular UK legislation. The MP may say "sorry but EU law require that - there is nothing I as an MP in the UK Parliament can do". If the citizen goes to their MEP the MEP may say "well I asked the EU Commission and they say the problem is not with the EU directive but with UK gold plating". Essentially every political representative passes the buck and nobody can prove (absent a legal case which finds it way to the CJEU) who is right.
This is a complicated issue. There are four main institutions in the EU: The EU Council of Ministers (made up of one government minster from each of the member states), the EU Commission (in practice the body with the most power in the EU made up of commissioners appointed by each member state), the EU Parliament (directed elected by citizens of the member states but having very few powers) and the Court of Justice of the European Union (CJEU) which rules on matters of EU law.
EU law is very important in the EU but not all decisions made by EU institutions involve changes to EU law. If it is decided to introduce a system providing some consumer rights, for example, then that can only be implemented by law (because a private manufacturer can only become liable to compensate a consumer if there is a law to that effect). But if it is decided to implement a standard for passports that does not necessarily have to involve EU law. The only bodies which issue passports are the governments of the member states so if the Council of Ministers resolves that they will all adopt a common standard for passports that may be all that is needed for the decision to be implemented. In recent decades the Council of Minsters has made a resolution (which does not change EU law) to adopt measures such as microchipping and later it has been decided to make that into a regulation which does change EU law. The key to understanding the Blue Passport issue (and whether the UK could have introduced Blue passports before it gave notice to exit the EU) is that the original resolution of the Council of Minsters on 23 June 1981 remained a resolution and was never made part of EU law by regulation.
There is a lot of horse trading in the Council of Minsters. One issue may be politically sensitive in one minister's country. A different issue may be politically sensitive in another minster's country. A minister who is accommodating to other minsters on issues which they feel strongly about will find other ministers may reciprocate. A minster who is perceived as always being "awkward" may find the other ministers ganging up on him. So there is a lot of political pressure to keep to what has been agreed in a resolution. On something like passports which are entirely under the control of states you don't need to make a resolution into a regulation for it to be, in practice, binding. The resolution was enought to ensure that all member states at the time changed over to Burgundy passports.
The fact that the requirement for passports to be Burgundy was "only" a resolution and never made into a regulation would not normally make much difference but it did mean that when new states applied to join the EU there was a degree of flexibility. A new member would normally be required as a condition of entry to have Burgundy passports but if they had a problem with that (e.g. a former communist state which was not keen on the colour red in association with official documents) then that requirement could be varied as it was not a matter of EU law.
The fact that the requirement for passports to be Burgundy was "only" a resolution also meant that when the UK government under Prime Minster Theresa May (in the middle of political turmoil in the UK over Brexit negotiations) announced that Blue passports (which had been talked about before but not given high priority) were an important symbol of national sovereignty which Brexit would allow, the EU Commission immediately shot back saying to the media that the UK could have changed the colour of its passports at any time if they felt strongly about it and it was nothing to do with Brexit. The EU Commission was able to do this because it had been tasked by the member states to conduct withdrawal negotiations on their behalf (and so presumably were empowered to vary the 1981 agreement in respect of the UK - it being only a resolution - without formality).
So could the UK have introduced Blue passports before it gave notice to exit the EU? In my view, not really. None of the member states which introduced the Burgundy passport has ever tried subsequently to change the colour to something else. If the UK had tried to do so it could not have been taken to the CJEU (because the 1981 agreement is a resolution not a regulation) but the political reality is that the working of the EU relies on minsters abiding by resolutions - if the UK had tried to go back on the 1981 resolution without agreement there would have been retaliation in one form or another.
That said, the question asks about "enactment of change in laws" in the UK and technically the UK government issues passports under the Royal Prerogative so no Parliamentary law is necessary to start issuing Blue passports. It is something which the UK could not do (consistent with its 1981 commitment) before exiting the EU but because it happens to be one of the constitutional areas not yet brought under Parliamentary control in the UK, changing the colour of Passports does not require any change to UK law.
Changes "in the pipeline"
Although not enacted yet, and so strictly outside the terms of the question, the following changes are expected soon.
The Animal Welfare (Kept Animals) Bill has been introduced though, obviously, it is not an Act yet. It will, among other things, ban live exports. This was not possible to enact before as EU law does not allow member states to ban live exports.
Regarding Imperial Units the UK government has said that it will review the EU ban on markings and sales in imperial units and legislate in due course - no Bill as yet. It is well known - e.g. from the Metric Martyr's case - that (subject to certain exceptions such as Beer and Cider) sale of goods by imperial measurements is not permitted by EU law.