Taking a quick look at BA schedules, I was surprised to see that they actually operate more flights to continental europe destinations out of BHX than MAN, many of them by A319.
Surely BA are being squeezed by EZY and KLM-UK on MAN-AMS. The many pax transferring at AMS will naturally choose KLM-UK, and for the rest there is a frequent cheaper alternative from LPL ie EZY.