I had the same dilemma but my appointment for the booster & flu vax was when I would have been at the airport , I was also concerned that the vax might show up on a PCR test , so called the Nhs vax centre & they said they don’t think so , I’m not taking the chance , can get it once I’m back lol
you’re a lucky man , my friend from the US missed his visa extension due to being in hospital for 2 months (close to death at 1 point ) he ended up getting 6 months in the Bangkok Hilton before getting deported ! He’s black listed for 5 years (3