Anothertyke
I think your Mandy Rice-Davies analogy is very good! I am also inclined to be sceptical about BA's "support" for T5. Certainly the entry into LHR of easyJet would put a rocket under BA's inefficient short-haul operation, potentially undermining their hub operation unless they could reduce costs to compete.
I would hope, however, that if R3 doesn't go ahead the competition authorities would be on the ball so as to ensure BA did not abuse its dominant market position. But it's difficult to see what the remedy might be in that situation.
Leaving all this aside, I do think that pressure needs to be brought to bear on HAL to sharpen its pencil on R3 costs. The CAA has hitherto proved useless in keeping HAL's costs down, so a bit of early pressure from BA and others will do no harm.