The market does support legacy long haul but struggles on British registered legacy short haul. Though last time I looked MAN still have Air France, Lufthansa, KLM, CSA etc etc.
The whole point though is that AF, KL, LH, CZ etc.... are all serving their main hubs from MAN.... as is BA and to a lesser extent bmi. A British registered legacy carrier operating short haul flights ex MAN would be relying mainly on point to point traffic, up against non British legacy carriers having the advantage of operating to their hubs.