I am not sure how in a totally integrated free market world, a country like UK can compete with US or China. The amount of funding US can put into new areas like green energy and the internal market and capital it offers is something UK can't compete with. This is why UK origin founders are one of the biggest cohort of immigrant unicorn founders in US.
Although, I am not sure where a Brit would move to, if they want to have similar social systems. Most of the EU also seems on the path to decline as well.
Also, unlike say a German or a French expat, a British expat would have little problem moving to US, Canada, Australia or NZ as there is little language or culture barrier, so if those countries do well, why won't a skilled Brit just moved there.