Actually Martyn it still is supply and demand. You will have sectors of people who will travel on any airline because of cost, but you will also have people who will travel on the airlines that they feel comfortable with. So in the case of BKK, UK residents and Australian residents may say “I’ll travel BA or QF only” and Asian “I’ll only travel Thai Eva”.
Binmans suggestion that BA is catering for people around LHR is unfounded. What about the people of South Wales that have never had an air service to LHR or the South West. As for people in the north and Scotland they have the choice of taking BA through LHR to a destination or someone like KL via AMS, widening choice.