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.
Agreed, this is the most likely outcome in the short term.
In the long term there are going to be unexpeted knock-on effects. Even a modest definancialization of the American economy makes it more robust to demographic changes and the end of the population-driven growth era. Lowered imports from abroad (especially china) and decreased consumption writ large means lower carbon footprint. Maintenance of airplanes getting more expensive means people fly less (already happening), again, lower carbon footprint.
It won’t cause the economy to collapse with alarms going off everywhere.
It will instead be a quiet, slow and sustained shrinking of the economy.