BA wont interline the baggage.
It doesn't sound thru-ticketed becuase you would most likely have two paper tickets - Unless BA has an e-ticket agreement with Thai.
If it's two seperate tickets it is indeed much cheaper for the airline to only take your bags as far as they take you.
At my airline we've just been told as of March we're not allowed to interline bags for pax on seperate tickets. And not onto some airlines at all - thru ticketed or not