As someone who lives closer to Manchester than to any London airport, when travelling scheduled/long haul I prefer to fly from Manchester, of course. Now, if I choose our "National Flag Carrier" (BA) they will always fly me to London first and then onwards. So doing represents (on a round trip) four wholly unnecessary passenger movements at (say) Heathrow, per person, per trip.
Without any statistics whatsoever to back this - I wonder how many people are, in fact, in as easy, or easier, reach of MAN than LHR and who therefore contribute (to the extent of 4 x movements) to the capacity issues at LHR every time they are so routed. And to what extent greater use of provincial potential hubs like MAN for direct flights would in fact reduce the LHR problem.