Would this site give an indication of UK Embassy availability:

https://www.book-consular-appointment.service.gov.uk/TimeSelection?location=53&service=15

 

If so, it seems that dates for slots aren't released for 6 weeks in advance, and that they go pretty quickly when they do.

 

I'm not going to marry until September next year, when I have a month in Thailand, but looking at this, I can't finalise my travel until 4-6 weeks ahead. Or is availability better than this suggests?

They only offer affirmations on Tuesdays and Thursdays. The appointment slot availability grid is a bit misleading and should highlight this. I went last week for address affirmation for driving licence. Had no trouble booking in advance (and changing dates due to changed travel plans). Just looked now and appointments were available for less than 4 weeks ahead (13th December availability). Suggest that you book at earliest for the most likely date needed. The new offices are very nice, staff helpful and professional. Easy to get to as the building is connected to BTS Saint Louis by sky bridge walkway. I was in and out in less than 15 minutes.

 

Many thanks.