Isn't it that LON-USA flights are the most profitable ones?
Even if they lose most traffic to the Far East/AU to the Gulf carriers/CX, there is very little alternative when you travel to the States:
- UA is one big crap
- DL is now marginally better than BA, but their FF is much crappier than BAEC (just visit the DL forum)
- AA (on B77W's) and US (on A330's) are superior to BA in premium cabins, but they are (or - for US - will soon be) part of the JV, so BA would get your money, anyway

- VA in still unallied and cannot be a serious alternative
They simply do not have to try harder