I have a through ticket booked by BA but on Qatar and Bangkok Airways return Heathrow - Doha -Bangkok - Koh Samui return. Will I have to go out and recheck in at any of these airports?
Given that Bangkok airways is not part of the one world program, I'm not sure if you may have to do a change in Bangkok to Koh Samui as I don't recall if Koh Samui has immigration or not. However, Doha-Heathrow should definitely be check through.