How much would it have cost to keep 4 777 there for routes to say JFK, Singapore, HK for example given the full front end cabins for carriers on these routes via mid east or direct ?. I agree with your comments re short haul by the way, but think they missed a trick long haul. No way back now though.
It's not only a question of cost. Apart from anything else, BA doesn't have enough longhaul aircraft at present to develop new routes from LHR, hence many of the ex-BD slots it inherited are being used on Mediterrenean holiday routes for now. So there would be no ability to base 4 B777s at Ringway, even if desirable.
As for MAN-HKG, provided it was in the interests of both carriers, probably the best way to to do this would be to codeshare with Oneworld partner CX. Ditto with SQ for MAN-SIN (different alliance membership doesn't necessarily rule this out).