Scotland is going to leave England+Wales
Scotland is going to leave England+Wales, so is Northern Ireland, I'm just less sure how that is going to work it self out.
Scotland (5,3 million) has a population close that of Denmark (5,5 million). They can easily be on their own as a independent country.
Northern Ireland has the population of 1,8 million. That is close to being the same population as Latvia (1,9 million). This is good enough to independent nation and to join the EU again proper. Not saying the change is going to be easy, it never is. But it can be done and I think it is going to get done once the UK is out of the EU (for the next 60 + years as England + Wales).