Fellow PPruners:
isn't it likely that the BA aircraft had a bunch of cargo that would have to go anyway? I know that when I was actively flying I flew trips with 2 people on board, but a lot of MAIL and other cargo in the holds. In the US, mail contracts must be met.
I've even heard that most of the costs involved with flying across the pond are covered by cargo. Pax are the icing on the cake.
jon