The proposed 3rd runway for Heathrow was originally to be configured for smaller aircraft ie A320 and 737 families and was supposed to give 2000m TORA/LDA, which could be provided at Wisley, however if you look at the western end of the runway, this already ends in a steep bank down to the level of the adjacent A3 - in fact you can see it travelling east on the A3. I don't think the CAA would licence this without considerable earthworks in order to provide a level area at the end of the runway.
Now if you were to look a few miles further west, there is already a licenced, ILS equipped runway with a paved surface of 2400m positioned adjacent to the M3 and the main railway line to Waterloo. It wouldn't take too much to equip this airport with a link to the M3, thereby allowing quick access to Heathrow.