Be aware of the impact of the strike on processing..............it could cause delays in the UK, as well as processing through Thailand. 10 weeks is the advised processing time in the UK. You will need to see if they are accepting fasttrack applications:-
https://www.gov.uk/get-a-passport-urgently#:~:text=You can pay to get,to 3 weeks in advance.
Recent experiences of turnround times are:-
30th January, back on 25th February
20th February, back 3rd April.
28th March, still waiting
11 weeks is the current advised processing time.
Overseas British passport applications
How long it takes
You’ll need to allow up to 11 weeks to get a new passport.
https://www.passport.service.gov.uk/overseas/information/thailand/renew/adult