I know the feeling, had the same chance at Paris, turned into a nightmare getting back to CDG in time. Had already been to Paris and just wanted a fast refresh of seeing the eiffel tower and arc.. Wish I hadnt.

Odds are if COVID did not exist and you had nothing to carry around all day ( could store all in locker at airport but passport/wallet) at best I could see you seeing nearby Windsor Castle.

Flying out of Heathrow is a chaos of different terminals and nothing easy no matter how many times you've been.

If you did store stuff fast, took the express train to London, You might get a 1 loop double deck bus ride and some bad fish and chips.

Any other more optimistic opinions with/without covid?