It is technically illegal, but also standard practice. I've been teaching here for 20+ years and every school I've worked for has expected new teachers to start work while their work permit is being arranged.
Fortunately Thailand is moving on. My son's school has just informed us that scout uniforms are no longer compulsory, just a scarf/toggle. No required haircuts either. It's progress.
Those with a wife already in the country and on the path to ILR and citizenship will hopefully continue on the same pathway?
Raising the level of English required will make it much more difficult for Brits to get settlement for their Thai partner, as many Thais struggle to pass the current A1 level test.
All UK visa decisions are made by UK civil servants working for UKVI.
VFS have NO part in the decision making. They just take and check the applications before forwarding them on to UKVI.
Why? Since 1984, UK passports have been issued by HMPO which comes under the Home Office. It has nothing to do with the FCO.
HMPO, however, should answer emails!
I think it must depend on the local Immigration Office. I live in Rayong, and they do not require a new TM30 after coming back from abroad. I've had the same one for 4 years.