If you are flying regularly to Thailand why not look at doing return fares FROM BKK issued locally in THB. Trailfinders or any other major QF Contract holder in the UK should be able to issue you with a UK-BKK oneway ticket to start your travels. If you then contact a reliable Agent in Thailand (STA, Diethelm, American Express etc.) they can advise you the fares ex BKK.
BA do seem to have Special Fares via Travel Agents in markets other than the UK. Only a suggestion........