Trade is bi-lateral. The UK’s trading partners were also harmed by their decision to leave. Short term, it’s just simply a net negative for everybody. Long term (decades), the UK will have a long run growth rate lower than the rest of the EU. Granted, our lack of a crystal ball means that we can just debate outcomes ad nauseum.

If I recall correctly, the UK changed the way in which they compute the government contribution to GDP (going from G being just cost to some estimate of the output). This caused both the COVID drop and recovery to look steeper.