If I was a betting man I'd bet the airlines have told them to do their own checking, etc..and not expect them to do the Thai government dirty work for them...the corrupt useless w⚓'s...
As is typical of this lot ( Thai government) the rules change almost hourly...none of them could run a bath let alone a country..what an utter shambles
in which case you need to apply from the UK via the Thailand Embassy in London...the application is done via the Thai website and from memory the requirements were a satisfactory police check; a doctor's certifiticate stating you are free of certain diseases; proof that you have sufficient funds in your uk bank account and health insurance..unsure of the amount but it will tell you on their site...
For what it's worth if I knew then what I know now I'd be using the O visa route and not the OA...much simpler
I would fly direct Bangkok to Heathrow or if you want a stop and maybe cheaper flight go via Helsinki or Amsterdam...avoid the middle east...once landed in England catch train or coach to Scotland and then do your 10 days quarantine which can be cut to 5 days if you have a test which proves negative after 5 days..