BHX is just too close to LHR and LGW, that's the sad fact. Long haul operators are not going to start a spearate service from BHX when BHX#s and LHR's catchment overlap so - i.e. Oxfordshire/Warwickshire borders, Morthants and Gloucestershire.
MAN is considered better for long hauls like CX, AI, EK, RB, DL, AA, MH and SQ because Manchester ahs a far wider catchm,ent area i.e. northern England from Birmingham to Carlise and Newcastle, Wales and Scotland.
Iy's nor rocket science.