@Dave *********
it's hit and miss with any airline, as to whether you will be required to have a flight leaving Thailand within the number of days you're stamped in for, if you're worried just book a cheap flight to a neighbouring country, or just chance it!! If they won't allow you to check in, when you arrive at Heathrow, just step away from the check-in desk, and book a flight there and then, they would accept that no problem.