I looked into the deed poll UK name change as the easiest option, but we live in Glasgow and Scots law is different. Only people born or adopted in Scotland can change their name here. The Passport Office doesn't insist that we travel to Thailand, only that the name in the Thai passport must be changed. The first step in this process is to register our marriage in Thailand and change the name in the house registration document, then get a new ID card. This can apparently be done from the UK by giving someone in Thailand the power of attorney to act on our behalf, but it's a complicated process and going to Thailand would probably be easier. Also, considering the long wait for an appointment at the London embassy, it would be much quicker to get the new Thai passport in Bangkok.