Of course there is great overlap between the two in reality, but at their core the two legal systems have different philosophies. Hence why you end up with situations where the UK may be more flexible about things like separation.
There was a huge political situation after WWI about that, but in the end 5 countries agreed to allow the counties in the north of Germany to choose if they want to belong to Denmark or Germany.
Sorry, I need to correct that. Not just the govern, an overwhelming majority of citizens in this country have no desire to modify the current regional system. At least in the direction that separatists want.
While Madrid's handling of the more recent Catalan independence campaign has been pretty inept the Brexit vote sets a pretty strong counterexample to "just call a referendum, you'll win, it'll be over for 20 years". Scottish independence is a good example too, that's hardly gone quiet...
Scotland.