Could the devolved UK administrations legislate directly for Brexit?

Question

The UK government is currently trying to pass a Bill to put into practical effect the departure from the EU, by incorporating existing EU law directly into British law at the date of departure.

For various reasons this is politically contentious, one issue being that it legislates in areas that are devolved to Wales, Scotland and Northern Ireland. This means that the Sewel Convention applies; either the devolved administrations have to agree to this, or the UK Parliament has to take the politically undesirable step of going ahead anyway.

The administrations in Wales and Scotland aren't keen on agreeing to the Bill as it stands, though there are negotiations underway to resolve this.

Would it be feasible for them to instead pass their own legislation to address the issue, e.g. by incorporating EU law that affects devolved matters directly into their own legislation? In particular, is it legally possible, and if so is it practically possible or would it just be too complicated?

Of course if the UK Parliament decided to go ahead with the current Bill anyway, it would override any such devolved legislation, but it'd be hard to argue it was necessary.

Answer 1

It appears the legislation is now being proposed by both the Welsh and Scottish Governments, though it's not clear whether they are bargaining tactics or because they seriously expect to make them work.

One reference also suggests there is a fair bit of legal uncertainty, particularly in drafting a bill that was actually within the legislative competence of the respective assemblies. That could make it hard to pass in the first place, or lead to it being struck down by the Supreme Court.