TG agents at BKK are accustomed to sending baggage through (due to ICQ), and since TG and BA have an interlining agreement, the agent can technically send it all the way. You should casually approach the check-in counter and mention that you are traveling to LHR on two separate PNRs and would like to send your baggage all the way to LHR. Show them your BA ticket and see what they say. You may get lucky, but don’t be disappointed if not.