Many of the passengers using Manchester are coming from areas all over the north as it has such a large catchment area for long haul flights. The choice for many people in the north is Manchester or Heathrow.
These people will choose to use a shuttle service from the likes of LBA and connect at LHR rather than drive to MAN once R3 is built. People are already using the BA flights from LBA rather than going to MAN. Anyone that has had to slog over the M62 in winter in the early hours will know this is a preferred option.
It is simple supply and demand. The airlines spend a lot of money researching new routes and if they choose LHR over MAN then that should tell you everything you need to know.
The fact that the Manchester Airport management and enthusiasts are so worried about the effect R3 will have shows that it will be successful in providing for those in the northern regions.
This thread seems to be more about Manchester than Heathrow lately.