I still think that if the Union is to be dissolved then all voters in the Union should get a vote.
I see your point, but not sure I agree. If a country wants to secede from a political union then it up to the citizens of that country to make that decision.
Could you imagine a Soviet Union-wide vote on the granting of independence for its consituent countries going the way of the secessionists? The Baltic states, Ukraine etc would still be part of the greater Russian empire because the Russians who made up the bulk of the Soviet populace would never have voted for it - why would they?
If such a voting sytem were introduced anywhere in the world it would always go against those seeking independence and instead go the way of the larger union as it has the larger population.
If Scotland wants independence then they should vote on it without interference from beyond its borders, and the same is true for England and Wales.
Ps; I'm not Scottish.