And keeping Scotland and Wales is seen as a reason to stay in ?Thing is mate, it just requires a bit of thinking ahead. Even closer to home, it sounds very much like a Brexit would prompt Scotland to leave the Union. Would Wales want to leave too? What would happen in Ireland? Would it require a 'hard' border between north and south? Would the north want to remain in the EU themselves and if so, would that precipitate a unification of Ireland, with all of the ramifications of that.
I mean, believe it or not, our economy has actually been in growth for a few years now so by most laws of economic cycles we're probably due a recession before long and we have an environment where lots of knee jerking populists who love blaming other people for their problems are gaining ground all over the continent, and the prospect of a country leaving the EU that would almost certainly cause sufficient short-term economic trouble to kick us into that recession that we're due. All evidence suggests that it's in exactly those times that those aforementioned populists gain even more ground, which may well result in large chunks of Europe being governed by racists or socialists, which worked bloody marvelously the last time it was tried.
Deffo out from me if that happened.