Westminster would (and should) simply refuse to assent to another referendum, at least until negotiations are completed, whenever that may be. If Holyrood attempts to organize a referendum on its own and declares independence on the basis of that vote, there's more than sufficient national and international precedent from
the last time a country tried to unilaterally secede from British rule to declare it illegal. And the prospect of the EU negotiating with a Scotland that has unilaterally declared independence would spook out the Spanish to such a degree that it would be a non-starter for that reason alone.
Heck, I'm willing to argue that Gibraltar is more likely to become an independent Commonwealth Realm sooner than Scotland would become independent. Or, at least, that's how it
should be.
Poll
