And look at the huge losses AF/KL and LH are currently making compared to BA.
I believe BA currently also operates to NRT and HND. But more to the point BA previously operated to many more Far East points than they do now - TPE, JKT, KUL, BNE, MEL, ADL, PER, AKL, CHC, CMB and probably others I've forgotten. All those destinations ceased because BA couldn't compete with other carriers on the routes and as a result each was making huge losses. Are those destinations suddenly going to become viable again and be added to BA's route map just because of a third runway ?? I don't think so. How many other viable new destinations from LHR are really out there ??
I don't doubt the need for a third runway, however the days of BA flying to everywhere on the planet with a long enough runway are long gone. More likely the extra slots would be used to add yet more flights to current gateways, especially those where their Oneworld mates are dominant.