you overthink it.If, for example, the UK imports chlorinated chicken (as just one example), I can see the EU being very wary over anything they might want to import from the UK which contains chicken. The obvious thing would be for the EU to try to stop any chicken-containing imports from the UK. Even if that cannot be achieved by use of the law, the population of EU27 might choose to avoid UK imports.
As this process spreads across the various foods and regulations, I suggest EU imports of UK foods will drop further and further. There might be some specific areas which are not affected very much, or at all, but all too many will be impacted.
we have already had chlorinated chicken fillets (tumbled chicken had chlorine added to kill germs).