Starone - I think you've nailed the problem on the head; BA's cost structure is such that they cannot profitably fly most long-haul routes from "regional" airports. Of course they're a business meant to make money, but I find it hard to believe that there isn't enough business to fly some long-haul routes from either BHX, EDI, or more from MAN, considering the catchment area of these airports. Apart from the already-mentioned fixed costs problem, BA seems to consider that if a route doesn't justify a daily rotation, then it's not worth doing...
Most of us will agree that the problem with BA is that nobody can make any sense of their current (or previous) corporate strategy.
Cheers
P.S.: Air Tahiti Nui's capital actually includes the local French Polynesian government, so I'd be surprised if they didn't get some financial incentive to fly to CDG (but they seem to be getting very good loads on the route anyway).
EZY-wise, my source was their website!