Makes little difference tbh using a prop, they operate at the same speed as jets on finals so not a massive issue. If they can make it work so be it? That's a free market, the mechanism to force larger aircraft would cut into routes available, LHR serves way fewer than AMS, CDG or FRA as it is, competition is exactly what BA need. Lack of it has given us easyJet seating on a network carrier, because LHR protects them.