I’m on a 45 day visa exemption. My British passport has almost ran out of pages and I’m applying for a new one at VFS. They’ll give me back my old passport, albeit with corner cut off, whilst British passport office prints out a new, blue passport for me.
Do I have to carry both passports through immigration at the airport when I leave, or, do I need to get the thai visa moved to my new passport?
TLDR : Answer Summary
If you are on a 45-day visa exemption and are in the process of getting a new British passport, you can exit Thailand by carrying both your old passport (with the visa exemption stamp) and the new one. It's recommended to show both passports at the airport. However, if you manage to extend your stay with a 30-day extension and you have your new passport, in theory, you should transfer your visa stamp to the new passport.