I would assume there are very few people with MPs using T5 B/C transfers, as domestic flights go from A gates, and most people that live in the UK are unlikely to be bouncing from country B to country C via the UK without returning home.

Complain to BA, they're the person you bought the ticket from, and they're Heathrow's biggest customer. They need to champion their passengers if they want to hold on to connecting traffic, or IAG could simply buy heathrow (SRB would love that!)