ATNotts: I don't find myself convinced that BA cannot make money flying from outside London - they just don't want to.
I can only speak with authority on the late 1970s, when I was in charge of BA's product planning on US routes. We tried everything to wring a drop of profit out of MAN and GLA (and very briefly BHX) - absolutely no chance. The routes were kept alive for reasons other than economics. Very little high-yield traffic and all the operational problems of maintaining schedule integrity with aircraft based away from home.