Apologies for the brain f*rt by forgetting another recommendation, up far too early to watch football:

The Re-Entry Permit.

This allows one to "bank" their existing visa (real) stamp. So, in theory you could utilize your one, single-entry visa arriving SBIA/BKK from the U.K., enter Thailand, then (at the airport) apply for a single re-entry permit (form TM.8+photos+copies+1,000 THB), prior to flying BKK-CMB; be sure to give yourself some extra time to accomplish this procedure, although you could do it immediately after clearing arriving Immigration.

Having the re-entry permit would relieve any issues boarding/flying CMB-BKK, and the visa could be further extended inside Thailand for another 30 days.

Although this approach may not give enough time, based on the outside limits mentioned (two weeks + 3 months).

Again, sorry for not mentioning this previously.

http://www.suvarnabhumiairport.com/e...-re-entry-visa


edited to add: While I've personally used a Re-Entry Permit to "retain" my current stamp-in date(s), I did have a Non-Imm B visa (and was preserving my 90 date stamp which extended my visa from 12 months to 15 months). So I am not sure, without conducting additional research, if this is an option for someone with a real Tourist Visa. I'll try to get some sort of idea on applicability and follow up.

follow-up: a cursory review of other forums confirms that a Re-Entry Permit would be an acceptable option for you if you have a real Tourist Visa.