If BA really wanted to maximise their market share of Belfast via Heathrow to the rest of the world, the best solution is their on metal into T5 from Belfast.
BA lost a fortune on the then Shuttle in it's final years. I believe the main issue was that because so many passengers were transitting onto long haul, it was long haul that was allocated the bulk of the ticket revenue. Hence massive loss for short haul ! This is similar to BMI and the Glasgow route quite recently, with every single STAR partner codesharing on the BD014 and the route still being knee deep in red ink at the end of the FY.
As things stand, BA get the connecting passengers from EI without sustaining a massive loss on their short haul network.