says, first make sure your name is shown on the Thai birth certificate and get it translated into English. You apply for the baby's UK passport via HMPO at VFS in Bangkok or Chiang Mai. Fill in the application form and check what supporting evidence is required. You will need your original long-form birth certificate to show you are British Otherwise Than By Descent and the baby's photos will need to be countersigned by a upstanding Brit who knows you! You can use an agency if necessary. I can recommend one but many others will too. It's fairly straightforward to do it yourself, if it's easy for you to get to VFS.