Be aware unless you are a British national: I am French (not tourist, family in LOS) and had a flight from Bordeaux to BKK with a 4 hours transit (no airport change) in LGW on Dec. 23rd; all paperwork OK, PCR test negative, PLF 24 hours before: I was denied boarding the plane in Bordeaux (by the french border cops saying I would be rejected by the Brits), thus also loosing my connecting flight: the reason?: no IMPERIOUS(!) reason (i.e. living in Britain, relative death, ...), a transit is not one; of course NO NOTICE from the British authorities, agency or airlines...I had to urgently book a costly KLM flight with a stop in AMS... No refund of any kind possible, they say all my fault!!!... 1000 euro vanished!...
So up to you, but for me UK is a NO GO!...