BA has tried operating a dual hub at LGW before ("the hub without the hubbub"). It didn't work and BA won't be trying it again.
It is true there is a fair amount of slot-sitting at LHR but BA has said that only about a third of the slots gained from bmi will be converted to long-haul ops as there is an optimal overall balance of 1/3rd long-haul and 2/3rds short-haul.
It is also true that there has been nothing stopping BA launching new routes to Asia if it really wanted to do. What doesn't help BA in Asia is:
a) LHR's position in Western Europe means it is not placed to pick up connecting trafffic from mailand Europe like transatlantic; and
b) Lack of local partners to provide onward connections and distribute BA's flights in the local market to private and corporate customers.
BA is evidently making a push into Asia with Seoul and Chengdu but it has always been clear it needs the support of local partners. It claims the codesharing with JAL and Tokyo routes is already helping to drive up traffic.