MME - LHR
Routes have to pay for themselves. So what on earth would persuade anybody other than BA to go to LHR? BA don't operate the service because other services pay more. And don't forget, we are talking about transfer passengers and not point-to-point (Darlington/Middlesborough to London) travellers, as these are probably better served by road/rail transport. There's probably a greater likelihood of Emirates arriving with an A330 than BA in an A319.
PM