You could not do MAN-HKG-MAN in a single 24 rotation so you'd need more than one aircraft positioned at MAN to service the route. It's just not efficient to set up a small base at MAN when there is much more higher yielding traffic ex-LHR (about 120,000 pax fly between LHR and HKG a month).
BA has had to learn hard lessons in the past from growing its asset base ahead of revenue and the reality is it has a very lucrative premium market in London and is much better being focused on that market.