soi3eddie

It seems to be that way. Unfortunately. For now.

Might just possibly work with British Airways heading to the UK but I wouldn't want to be on the wrong end of a refusal to travel from a foreign airline especially as it would likely stop the whole OP family's trip. Have you actually done this?

Sorry I meant at check-in (rather than boarding). The check-in agent would almost certainly not accept any expired passport.

Agreed with above however I would be wary of some airline jobsworth denying boarding for an expired British passport and his Thai passport not having a British visa. I know that UK would not reject him entry however one never knows how airline staff will handle it. Best to return before British passport expires.