The airline might not board the OP if he try this.

As I understand it, the OP is traveling from the U.K. to Thailand with a British passport, and is staying for ~ 12 hours before departing. I cannot see how an IDB is even a remote possibility?

Now when the OP flies from CMB to BKK and plans to stay for 60 - 90 days, he might run into some issues assuming he does not have a real, tourist visa.

AFAIK, a 30 day visa Exemption can now be extended for another 30 days, inside the country. This used to be seven (7) days. I have not seen, nor heard of, any first-hand, verifiable reports of this new 30 day extension being an issue, save in some unusual/extenuating circumstances.