You certainly will and by any airline flying to Thailand from LHR. I've seen people refused at the boarding gate after check in missed confirming their onward travel.
Exactly, I cant understand why people complain that they have to show onward travel when applying for a tourist visa or think they're entitled to stay as long as they like, it's one of the easiest countries to obtain a tourist visa, much easier than the UK or the US.
TBF the OP isn't breaking the law, it's not his responsibility to register himself unless he owns his own property. Also if the landlord has never rented to a foreigner he most likely would not know about a TM30.