LHR is currently the gateway to the world with more people passing through it's terminals to destinations outside of these isles than any other airport on the planet.
The airport handles pretty much all the worlds major airlines with little exception now that the open skies policy is in force. LHR wants more destinations and airlines and airlines want more slots and thats a fact.
Look at LH. The have bought BMI, not because they think it is a good addition to have to their business but because they have the second highest number of slots at the gateway which makes it an extremely lucrative airline to have under your belt. The stupid prices paid by airlines such as CO to gain just one landing slot per day also demonstrates this.
If LHR is to get it's third runway which lucks increasingly likely now, I would be EXTREMELY surprised if that runway isn't full within months of operations. Its like a sale at Ikea, airlines simply cannot resist the temptations of huge profits that LHR brings to their books.
The new runway will be shorter and so will most likely be used for mainly European flights and medium haul flights with I'm guessing the 767 as the biggest use, may 747s/777s/A330s to Cairo, Larnaca etc. which will mean the runway will be very busy anyway, its the main two runways which will probably have room for more flights which will probably mean these slots will be in higher demand with over 12000 feet of runway at the world's premium hub available to airlines from as far away as New Zealand which will make interesting reading when the allocations are published for who gets what. I'm guessing BAA will be working with BA to make sure it gets the most slots available.
On the BA/LGW/LHR matter I believe it makes excellent business sense for BA to move to a single 'superhub.' EK have shown how effective this can be (but they will become a multihub soon I believe then the new airport opens?) Other airlines have also proved this success. Britain is a country that is very price sensitive, even when we are enjoying years of prosperity and we are not the best with brand loyalty like the French are with Air France and Germans with Lufthansa etc. so operating two or three large bases, whether near or afar within the UK isn't feasable as we probably have the biggest price wars between airlines than any other nation I that I can think of! Streamlining their business will only make the stronger, reduce costs and help them compete better with other airlines that seem to be catching up.
R3 seems to be the equivalent of 'match.com' as BA and BAA will start a new romance is creating one hell of an airport with one hell of an airline and to be honest, if the IB/AA deal goes through there will be little LH or anybody else can do at LHR as that base will definately be covered!
(This is all only thearetical but if it were to happen I doubt BA would do badly out of it)