AFAIK, to paraphrase the rule set we have been given:
Crew member, Base in UK: rostered positioning back to UK at end of block of work you are exempt from self isolation.
Crew member, Base in UK: commuting/travelling back into UK as passenger (including return from overseas holiday for UK resident crew) - you need to self isolate, though you can break this quarantine to travel to work directly to perform company
rostered duties only such as sims, operate a flight/trip........