SSK has rightly identified the HUB (sic) of the problem. No matter what we may think LHR is BA's hub and it is constrained by its capacity. It is just not practable for them to move on mass as there key market is based on LHR. Yes BA have also sub hubs at LGW and LCY but these two cannot connect into the major hub of Heathrow. Its been tried but it does not work. The ramifications for lack of LHR expansion is the loss of business to other hub airports. One could see in the future BA potentially having to contract as the network cant expand with job loses etc etc. The big concern is the speed or lack of speed that will occur when a decision is made. A good example of this is HS2. It should have been built at least 10 years ago but its not planned yet until 2026 and I bet that will be delayed further.
Back off my soapbox........