> In this case, such a law in Scotland could potentially breach Article 3 of the Convention which prohibits inhuman or degrading punishment. The British Parliament can simply choose to ignore it, but the Scottish Parliament can't (unless the British Parliament chose to give it such powers).
Well, if Westminster choose to ignore the convention, I don't see why Holyrood would not choose to ignore Westminster?
This whole politics thing only works if we DON'T ignore stuff we all signed.
Holyrood derives its power from Westminster and Westminster can remove it. The UK Parliament is still supreme, but it refrains from making laws in areas that are devolved to local Parliaments (the Sewell convention). It could overturn the Scotland Act that created the Scottish Parliament. In theory, Scotland could ignore Parliament, but that would be akin to unilaterally declaring independence, which is unlikely to go well.
I think this is the point. All of these arrangements are ultimately by consent. Holyrood agrees to recognise Westminster's power, just as the UK agrees to recognise the ECHR.
This idea of 'power' is a complicated one. What 'power' does England have over Scotland other than the power that Scotland has agreed to accept (see Northern Ireland for what happens when one of the devolved nations is conflicted over whether to accept that power)? What 'power' does the ECHR have over any of its signatories other than that which they've chosen to accept?
There's a well-written and detailed exploration of the negative consequences of withdrawing from the ECHR here, and what is power except the ability to impose negative consequences for not following instructions?
> what is power except the ability to impose negative consequences for not following instructions?
Yes, I think power is exactly this.
> What 'power' does England have over Scotland other than the power that Scotland has agreed to accept
A monopoly on violence to enforce its territorial integrity as the United Kingdom. Withdrawing from international treaties is generally accepted to be within a state's general powers. Seceding from an existing state? Generally recognised by the current international order as unlawful and solely a matter for the state to resolve internally.
Would there be international condemnation if Scotland seceded and England tried to take it back by force? Yes, but doing so would embolden existing secessionist movements in, e.g. Spain or the United States, so those states (plus the ones who promote non-interference in domestic affairs) aren't going to intervene in any meaningful sense.
I think power must go beyond consent and incorporate the expectations and realities of the nature of sovereign state power on the international stage. The UK has the right to insure its territorial integrity, while the principle of self-determination is limited within that framework.
Well, if Westminster choose to ignore the convention, I don't see why Holyrood would not choose to ignore Westminster?
This whole politics thing only works if we DON'T ignore stuff we all signed.