PB the problems in integrating the ATC side for Northolt are not insurmounable, as one or two correspondants have indicated. The environmental/infrastructure issues will be the hurdle which will not make this viable. Youare right the airfield is on the A40 which is dual cariageway.
Most pax nowadays and for the forseable future will travel by car, therefore you already need to be looking at a major upgrade of a system already over its capacity at peak times. With the M25 already (now),in problems the extra traffic generated with associated rat-running would quickly bring gridlock to west London. Many pax use domestics to connect with International flights at LHR, there would be no economic sense in taking those pax to an airport 7 miles away from where they need to be. Schipol and Paris would love this idea however!. The rail link is sort of there, but on the wrong lines. London Underground's system is already stretched to capacity, and the Chiltern Line from Marylebone would not cover a sufficient catchment area for this proposal. We are after all talking in the millions of pax per year.
Most pollution emmisions immediately around airports is from ground operations, and again the pax's vehicles therefore again making this unsustainable in environment terms.
Noise, first you have to determine who lives 'close' to an airport. This can be done by noise modelling. Looking at the amount of housing required to be protected under a typical contour, you better have deep pockets! Plus you need someone to stand in front of people and say, 'We're going to increase the number of aircraft over your houses by 200(guess)%', we all know thats not going to be a vote winner. Now the same bloke saying 'This governments policy is to direct all future growth in the London Area to Stansted' is going to get quite a lot of votes, from those around Heathrow in particular. STN is the least worst option, a further runway at Heathrow, be it Northolt, Heathrow itself or Wisley
, is in my opinion highly unlikely, but are likely to be used as a bargaining chip to ensure major further developmet at Stansted.