For the moment they can't develop LHR as they have no slots, and they have no aircraft.
Plus, they loose a ton of long haul from the regions via KLM, AF, LH, SAS etc, look at the regional airports, they don't travel to Heathrow in great volume, they go via the extensive KLM network.
Add into the fact the Dublin has pre clearance for the USA, which I think is highly unlikely to be given to the UK due to the terror threat in this country, and I can see a ton of reasons why they would want to exploit the transatlantic from Dublin, plus it's already being discussed in one of the Irish Broadsheets.
Plus it's not just the UK, expanded EI into Europe and bring people via DUB for a quick transfer and pre clearance, it's a win win for BA if you ask me