Not sure BHX is really comparable to MAN. The former suffers by being within 1-1.5 hours drive from LHR, though admittedly doesn't have the shuttle flights.
The problem I have with MAN , as already pointed out above, is that it almost always costs more to fly from there than LHR. I have seen BA prices to JFK cheaper routing MAN-LHR-JFK than flying on the direct MAN-JFK. I also got quoted cheaper fares on EK and SIA from LHR than MAN.
As for QF, there's no point, EK/SIA took the MAN - Australasia market when they left with the strange notion that people in the NW would still use their services, even when they'd have to transfer at LHR from T4 to T1 after already travelling for nearly 24 hours.
I suspect MAN, in terms of L/H development, would be better off without domestic flights to LHR. If you cannot get a route in directly pax could still route via AMS/FRA/CDG. It;s also quicker to get to/from LGW by train from Manchester, even though it's via central london than on the comically late BA flight, cheaper too.