Q1 - BA did have a daytime flight back from HKG a while ago. Very unpopular. Business travellers want to minimise "dead" time and therefore favour overnight flights. If BA were to leave LHR any earlier, it would (1) restrict options for those travellers who prefer overnight flights after a full day of work and (2) sit in HKG for even longer than currently. Cannot leave HKG earlier as it would be way too early into LHR, and no point leaving at 3am...

Q2 - A380 - especially if you can get the upper deck.