Flights out of BKK are what they are. You need a covid free test within 72 hours of departure. Do not use airlines that transit through the Gulf states as the UK will deem you to have been in a red zone 10 days prior to arrival and you'll have to do the UK equivalent of an ASQ hotel stay. The cost is 1,750 GBP.
For full travel details to the UK see here:
https://www.gov.uk/guidance/red-amber-a ... ng-england
Thailand's amber, at the moment.
Don't know about getting vaccinated in the UK.
Getting back here, you'll need a Certificate of Entry which is obtained from The Thai Embassy in London. For that, you'll have to show 100,000 USD of health insurance that covers covid, your ASQ hotel booked if staying anywhere else than Phuket. If you're fully vaccinated, you've got the Phuket sandbox. I'm not fully au fait with that. The Thai Embassy website should have full details.
If you went back, you always run the risk that things may change drastically here whilst away that could put your return date back significantly.
I looked at returning a month or so ago and the above is what I came up with. I might be missing a few other things but hopefully others can fill you in.