Well considering you stated Bangkok to Heathrow and that you paid for seating it has to be BA as Thai, Eva, and BA are the only direct carriers and the other two have free seat allocation so no need to "pay for a seat" so therefore it must have been BA for your story to make sense, BUT BA's most expensive seat is £75 but that is only for the upper deck on the 747 and in club world business class, which by the amount of avios you were given I know you were travelling in economy , as BA's starting point for any business class complaints is 10000 miles