All conservatives members so far confirmed that it is an absolute No to second Scotland referendum. As per that, assuming that no approval is given for a second referendum in the coming months, is there anything that Scotland can do legally/constitutionally to get it despite the disagreement of Westminster?
Apart from the indirect but legal techniques of obstruction, there don't seem to be legal options.
The Scottish Parliament could pass all sorts of laws in the health, agriculture and justice departments, designed to frustrate the workings of the United Kingdom up to the point where Westminster might concede.
This obstruction process would be ethically questionable, but it seems to be the only element of power the Scottish Parliament would have, since there is no formalized way for Scotland to leave the United Kingdom (as opposed to, say, the formalized way the United Kingdom has for leaving the European Union).
As a comparison, a famous 20th century obstruction process was the Montgomery bus boycott. By collapsing the Montgomery bus system (40000 of the users suddenly stopped using the system, preventing over fifteen million bus fare tickets being collected), the African American community brought the system to its knees, while garnering country-wide and worldwide attention.
Ethics and risks
As to the ethical concerns, the fact that the Scottish Parliament has no say in activating a referendum, combined with the fact that the previous referendum was based on the premise that leaving the UK would likely mean Scotland dropping out of the European Union, a claim echoed by then European Commission President Jose Barroso, combined with the very high percentage (64%) of Scottish citizens voting to Brexit-remain, makes the obstruction process a morally-acceptable option to some. Politically-wise, though, it's much more likely that obstruction in the case of Scotland independence would result in popular backlash at home and abroad, not to mention the possibility of Westminster restraining the Scottish Parliament's power, in a move reversing part of the 1998 and 2012 Scotland Acts.
Brexit Remain vote and Scotland
Remain is in yellow, leave in in blue.
There is no formalised way for Scotland to leave the UK so for that to happen Parliament would have to agree because that is the supreme legislative body of the UK (Scotland has its own devolved parliament but that only has the powers granted to it by the UK parliament). So they can do nothing legally without the consent of Parliament.
The next election to Scotland's parliament occurs in May 2021. While the SNP will try to claim a mandate for another independence referendum now, to build support, they know that many of their backers in the General Election voted to try to stop Brexit.
Come 2021, if the SNP pursues independence as a single issue, in an election where the voting system does not encourage tactical voting, AND independence-supporting parties get over 50% of the vote, then they are likely to be given a 2nd independence referendum, regardless of what English politicians have promised.
In 1937 Ireland had a constitution which (although it went a lot further than the devolved authority today's Scottish Parliament has) subordinated it to UK institutions.
The new Irish constitution was enacted by a plebiscite independent of the Dáil (which set the mechanisms in place). The plebiscite was an in effect an act of popular sovereignty. A key element was that the senior Irish judiciary had personally all agreed to the process, ensuring that any court decisions would underpin this.
Britain had no option (short of military action to reoccupy Ireland) but to accept this situation.
If you pass a law saying it's illegal for a certain minority group to have human rights, should they (and the rest of the world) be OK with it just because it's legal?
It would be quite funny if you "capture" a nation, pass a law where they got no rights, and expect they should just be OK with it because otherwise it would be illegal. Kinda oppressive, where its outlawed to criticize your master.
Point is: No sense in discussing is it legal or not. If an entire nation wants independence, there is nothing that can or should, legally or morally hold them back.
In Kheldar's list of options for Scotland, I miss one:
Scotland could invite a foreign power to invade and liberate them. For example Russia.
I know it sounds weird (and the Russia example is even more weird), but technically: If a legitimate Scotish Parliament with a, say, 75 per cent majority asks for this (and it might base its decision even on an additional referendum) - could it be deemed illegitimate?1
This may sound like a joke (and actually it started as one), but interestingly the idea of foreign intervention seems not to be totally beyond (theoretical) consideration. A commenter said: "Indeed, the UN recognizes the right of people to govern themselves, Parliament has enough recognition to be considered to represent the people of Scotland, and we have witnessed recent 'liberations'."
An example for Russia having been welcomed to liberate a nation is Bulgaria (1877-1878, Russian war against the Ottoman Empire with Russia invading). Well, okay, one might dispute how welcome that war really was at the time (the part of the population that did not welcome it has been largely ignored by Bulgarian national historiography since), but indeed there were groups in what is today Bulgaria who actively faught for foreign powers to intervene so that the Ottoman Empire might retreat. Which indeed happened.
Can Scotland learn from Bulgaria?
Here is a twist towards reality: We do not believe that Scotland would really want to have a foreign power invading. However, only speaking out the invitation would already have an effect, namely on the field of public opinion. The UK could not ignore this clear vote to leave anymore. Moreover to be safe, Scotland could invite a distinctly small foreign power which does not really present a threat, perhaps San Marino or whatever. The power of the act would lie in its symbolism.2
1 Of course, it is hard to imagine that Scotland would call Russia for help. Perhaps we should consider other powers. Uh... Norway? At least they have some positive track record with conquering these islands...
2 Another twist would be not to invite a foreign power, but to declare war to it. And then immediately surrender. - I admit that this is not my idea. I heared it somewhere in late night comedy, although probably not applied to actually Scotland.
Legally, Scotland has been a part of the United Kingdom since the acts of Union, and there is no route provided in current law for Scotland to leave that union. It's no more possible than Yorkshire or London leaving the UK.
So it has to be a matter of moral pressure. The principle was conceded by the government of the UK, that if a majority of Scots voted for independence, it would be granted. A referendum was held in 2014. It was agreed by the SNP that this would settle matters for "a generation". Only 45% of Scots who voted, voted for independence.
That should have settled it until at least 2034. Although I accept it's possible for a woman to be born, conceive and produce a baby in less than 20 years, it's a quite a stretch to call less than twenty years a generation, let alone six.
And it's surely a part of being part of the UK, that a decision about foreign policy taken by a majority of the population of the whole UK is binding on Scotland as much as on Yorkshire and London. That decision is, of course, Brexit.
It would be reasonable to expect another referendum in Scotland in 2034 or shortly after. The Nationalists are indulging in "special pleading". In fact, they did not receive a majority of Scottish votes in the recent general election, and every other party standing was Unionist. So based on the evidence of that vote, they cannot even claim that a putative majority for independence exists today.
If they were really sure, I don't think there is any provision in UK law to prevent the Scottish Parliament conducting a purely consultative referendum, such as that which was recently attempted (and violently suppressed and prosecuted) in Catalonia. I do not expect that the UK government would emulate the government of Spain. This referendum would take place, and if there was a majority for independence, the moral case for refusing to enter into negotiations would be very weak. So why, I wonder, doesn't the Nationalist-dominated Scottish parliament do exactly this?
My own opinion, is that they believe they would lose again.
There is no legal base as long as the parliament doesn't allow for it. There is also no legal base for it being binding in any way (unless... etc etc), even if a referendum was allowed.
But, more importantly, there is absolutely no practical base. Therefore, no matter what, Scotland leaving will not be granted.
The 2014 referendum was a
posse charade. UK tolerated it knowing well that after the fear and uncertainity that was deliberately spread as well as the deliberate Brexit misinformation, it would almost certainly not get through (but they could say "We did ask you, you know!" afterwards). But even if it had gone through, they could have and would have ignored it.
There is no practical base because UK needs Scotland, fullstop. It is the same situation as with Spain and Catalonia -- very similar situation, Spain cannot afford to lose Catalonia.
If Scotland leaves, the UK (or rather "England") is factually dead. Scotland isn't just a couple of whiskey drinking shepherds.
Virtually all the UK's oil (and gas) is indeed Scotland's oil (and gas). A sizeable proportion of UK's gold is located in the Highlands. Scotland is producing huge amounts of (renewable) energy which, of course, the UK uses. It has one of the biggest potentials in the entire EU for (future) wind and tide energy, too. Where would UK submarines and nuclear bombs be located if Scotland is no longer UK?
Glasgow gone? Yeah, who cares, nobody needs machining and heavy industry. Edinburgh gone? Who cares about that silly tech stuff and them start-ups. Royal Bank of Scotland gone? Ouch. Alright, now, that one really hurts.
The UK will not let this happen, they cannot let it happen, since they cannot afford. The question about a legal possibility never arises. There is no way they'll ever agree to Scotland leaving (short of losing a civil war).