Suvarnabhumi

Yes Correct. Its ALWAYS cheaper to buy the ticket in the country of the 1st leg departure. Because the Agents/Airlines can offer local promotional fares. However the Thai local market is generally cheaper than the UK local market, but not always. If you buy a Thai Airways ticket in Thailand but LON-BKK-LON, they cannot offer any local promotions and will have to do cross border ticketing and charge the full UK fare listed in the UK reservation system, this will cost serious $$$$$'s.