Having just done France-UK-France (as a motorised tourist) I found it to be relatively easy. The hardest bit to organise is the test and UK paperwork going France-UK. But I do have the EU vaccine pass! I do not know if Bojo's mob have got the equivalence sorted out for UK vaccines going into France (everything seems to have gone a bit quiet, it may be sorted; I will have to check the current french smallprint). It should be noted that in France you now need a vaccine pass to get into restaurants, sports events, festivals etc. Since these all attract tourist money it seems likely that there is a solution out there! If anyone wanted to do a particular french BRM it is probably not impossible (but don't forget the masks!)
I say I found it relatively easy - I was stressed as hell before the trip, not to mention the isolation rules changing in between booking everything and leaving!