Easy jet are no mino to be ignored -
In fact IAG know threat very well and Vueling are the tool to attack and destabilize this market.
Easyjet tend to shy away from pressure quickly and at LHR you and themselves would be mistaken if only EZY will enter LHR, EZY will have their own fight at LHR not just with BA. Didn't take long to drive them out o Rome or at least a big slice of operations and be sure FR and Vueling will be there as well and FR may say no now but it will happen.
A very significant percentage of BA traffic is actually inter Europe point to point and the current disruptive forces of flexible fares operators including Eayjet pose more threat to BA than even the ME3 and competing alliances via their own hub and spoke systems in the London markets.
An influx of long haul capacity east and west will destroy yield especially up front.
The biggest problem is if easyjet were to interline with other carriers which is only a matter of time so certain when lhr runway opens if it goes ahead.