As Skipness points out, the reason BA maintained MAN-LGW for so long after they acquired Dan Air back in the 80s was to feed their longhaul routes out of LGW.
Over time many of those longhaul routes were moved from LGW to LHR (West Africa, Atlanta, Dallas, Houston and others). The remaining BA longhaul routes at LGW are now beach destinations, and many of them are now accessible direct from MAN on Virgin, Thomson, Thomas Cook and Monarch.
So the reason for MAN-LGW no longer exists. The recession, cost increases and APD were the final nail in its coffin.