EGLL has FAR too much of it's customer base out in Berkshire, Wiltshire, Buckinghamshire, Hampshire, Surrey etc. for this to work. You have companies that have spread out along the M4 corridor BECAUSE of LHR.
There are 60 years of commercial and domestic development based on EGLL. No one is going to change that. Perhaps it was possible 20 years ago to instigate a new international field in the South East but not now. Further, that new field was never going to be east of London due to the massive amounts of pax in the central and southern part of the UK.
Secondly, the development of 'PrivatAir' type operations will take load away from EGLL and free up capacity. Also, the development of CO using 75s from BRS and 76s (I think) from GLA and the expansion of MAN have reduced load from them, as much as the LCCs using regional fields. Yes, of course, much of this is new traffic but much of it removed from Heathrow.
Thirdly, Northolt has been discussed in here a number of times and the military keep a very firm hold on it and, IIRC, their flight paths do not intersect in a good way with LTMA. They are OK for the small number of operations that they have now but NOT for a mainstream operation. (I sit to be corrected) Not to mention the considerable number of wealthy people in that area who will successfully bellyache if Northolt were to be seriously put forward as the third runway. Personally, I think it was the obvious answer. Domestic services pax would never go to LHR and a rapid transit system could link the place to LHR. But it will not happen.
What will happen is that the status quo will be maintained. We have missed the chance to be an AMS, CDG, FRA and will just mosey along with dear old LHR as grotty as always. The objective of all concerned is just to slightly expand LHR but never too much in one go, so as to produce a really serious opposition. Slowly, slowly, they are catching the monkey.
Lastly, this is just electioneering by Boris. He knows that big biz will never allow LHR to close or be replaced. He is saying this because he is standing for an election in London. Therefore, there is no discussion to be had!