I was surprised to see that Boris is against a third runway too. I myself was previously for it, but it shows what a little research and an open mind can do as I'm now pretty much against it. LHR is poorly situated, surrounded by houses and motorways. If expansion is needed (which it probably is, long term) then LGW or STN should get a second runway as a much higher priority (along with breaking up the BAA monopoly) than a third runway at LHR.
What BA/LHR actually need is more efficient use of the existing runways. Mixed mode is the obvious initial step. Bigger planes is the second (and fully airline controlled). With Terminal 5 and Terminal East LHR will have sufficient terminal capacity going forward, it's the runway capacity that's already maxed out. BA should forget the "big twin" idea and concentrate on getting more A380's IMHO as I really can't see a third runway being built any time soon.