Way over here in Australia, and having left England to live here way back in 1951, my opinion mightn't count for much but I'll give it, anyway.
It took a long, long while for the European Union to form and become reasonably established. In the earlier days, before the number of countries which signed up grew, it seemed to be going very well. All, good, hardworking, responsible nations with solid economies - then, it seems, the rot set in as more nations got aboard.
I can sense the problems, mainly one of feeling swamped by an influx of new cultures, new people, many of whom do not appear to assimilate - or not assimilate quickly enough for the majority.
Separation might solve the problem short term - and create as much angst as satisfaction, I suspect. But in the long term it might not be a good idea to have England going completely solo. The days of the great British Empire, still largely extant when I was a boy, have long gone. Can the UK - which definitely needs lots of imports to keep going - compete with its exports in the world at large?
I suspect not. And if I'm right, Britain will fall into the status of a Third, then a Fourth-rate power in many a sense.
However, as I said at the outset. This is a long distance opinion of one who really hasn't kept up with what's going on in the UK or the EU. But you did ask the question.