[matthewheath]

> 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.