Fleet planning runs up to 7 years ahead, BA will have (in my view) a plan to move the totality of Gatwick long haul and much of short haul to shore up market share at LHR.
They were killed at LGW by downsizing and allowing easyJet a foothold. Given LHR is the only game in town for real money, they dare not allow that market share to fall too far. If Runway 3 is built, easyJet (UK) will come calling, indeed plans are affoot already at T4.