I now know why BA have dropped their prices so much on the BKK route – the new EVA Air seat.
BA used to have the best hard product to BKK as the EVA and Thai business seats were only angled lie flat. Prices used to be much lower than BA, but now BA have cut their prices and EVA are charging a lot more than they used to.
This significant loss of premium yield might have been the reason why BA are down gauging BKK to a 777-200. They are also bound to be losing revenue on other routes, and the more they lose the more they need to focus on a completely new business seat.