P&M Aviation still seem to be building and exporting flexwings, TLAC are doing okay, with British designs, into the British market at least - and Just Aircraft in the USA are doing reasonably well with variants.
But yes, the UK light aircraft design and build sector is tiny compared to what it once was. Personally I think that this has little to do with Brexit and/or EU membership, and everything to do with skills and aspirations. We stopped educating and training the sort of people who do that sort of innovation - the ambition became to have a safe job with a good pension in a big corporation.
Possibly the lack of big aerospace companies in, for example, the Czech Republic or the Ukraine, has been the main reason for their success in light GA manufacturing?
If we go to Germany, I would say that their success is in large part down to the Akafliegs, which have provided visibility and training in that direction, that has died out in the UK. So a small but healthy proportion of young engineers there have taken their careers in that direction. They illustrate that you can have both, but only if you have the right cultural structures. Britain gets the "big corporate" / "heavy metal" side pretty well, but not the small company / innovation / new product side that the Germans and Czechs do much better.
But I really don't think that this has anything to do with the EU.
G