Maddog, it makes sense specifically because of the LHR slots and transatlantic rights issue. Then there is the fact that very few (none that I can think of, but I stand ready to be corrected) of the international alliances include two member airlines from the same country. The possible exception might be within the US, where the sheer size of the internal market
may make this possible.
As for
anyway for the sake of jobs and competition leave them separate
bmi and Virgin don't have any current routes where they are in competition!