Killing frequency on UK domestic routes is potentially counter productive bearing in mimd they need to remain frequent to be competitive on point to point. Given BA are focussing on improving profitability on short haul and given the Shuttle runs do quite well, I don't see a lot of scope for change in terms of cutting back.
Why would you go via LHR? Well count the number of destinations BA serve in the US versus the number EI do!
That's why.