I don't think its "ridiculously" tight as long as the OP is hand luggage only and gets a boarding pass for the Thai flight so he can use flight connections.
It is risky, how risky, hard to say. I was at Edinburgh airport weekend before last, a couple of BA flights were announced as canceled whilst I was waiting for mine, no idea why, but IIRC one was the one to Heathrow right before mine. if that was the OPs he'd be in a heap of trouble.
No idea why he didn't book a through ticket but presumably to save money, now he's between a rock and a hard place, either spend a lot of money buying another flight and a hotel, or take a chance on a very expensive missed Thai flight.
I most certainly wouldn't be taking hold luggage if I stuck with the existing flights, that's for sure.