I would be very surprised if the LGW BA 777s go to LHR (though maybe one or two routes may move across). BA LGW long haul is profitable in its own right and BA has already spare long-haul capacity with parked 747s and the 787s and A380s will start to be delivered next year.
BA long-haul growth at LHR with the extra slots is likely to be very cautious and probably will not happen until the 787s arrive.