All they will ask is you sign a waiver indemnifying the carrier for the cost of a return fare, should you be refused entry by an IO. I often travel LHR-BKK one way, no Visa with Thai Airways, never been asked for anything, nor has any IO in 22 years asked me for proof of anything. A smile goes a long way, knowing some Thai even further.
Every Airline I travel with stipulates that the passport name & ticketed name must match exactly, Thai Airways have questioned myself previously on this matter.