I don't anything about UKs economy and how integrated it is with the EU. However, it probably is extensive.
Thats why theres a transition period to create a soft landing. Whether theres an agreement or not on the final arrangements by the end of next year, Brexit is a done deal.
There's also the issue of what the U.S. does. It is entirely possible the U.S. could put it's thumb on the scales by cutting a more favorable deal with the UK than with the EU, and that really would hurt because the UK and U.S. economies together are larger than the EU's.
The EU has already made it difficult by trying to negotiate what the exit terms look like - including the demand for continued financial support.
They’ve succeeded in exposing the nature of the EU - it’s more of a cartel, trying to centrally plan everything, than anything that resembles the “United States of Europe”.
There are fishing towns in the UK that have been decimated by EU regulations - dictating whom can fish what waters and how much. It’s not just because of immigration.
The “remainers” argument is that the UK will go into a depression if they leave, although this already hangs over the country and the economy is showing no signs of impending doom.
...I’d imagine PDJT would be eager to do a US/UK trade deal the second they’re free.