One quite important issue is the border. English and Scottish people will almost certainly never tolerate any kind of friction at that border. We've been moving across it seamlessly for centuries, families are mixed, businesses straddle the border, etc. If the Northern Ireland debacle has shown us anything, it's if Scotland were to leave and then join the EU that border would become an absolute political nightmare. T…
Another way to look at the Northern Ireland precedent, is that a compromise was found. People could argue that the same text can be reused.
At least England/Scotland border doesn't have the bagage of the troubles, but it'll still be a mess.