Salisbury Plain? Rail/road connections could be easily modified for north and south access to this vast piece of real-estate.
I only have to quote from Wikipedia to show that this will never happen (in a more definite sense than the third runway at LHR will never happen!):
The plain is famous for its rich archaeology, including Stonehenge, one of England's best known landmarks. Largely as a result of the establishment of the Army Training Estate Salisbury Plain, the plain is sparsely populated and is the largest remaining area of calcareous grassland in north-west Europe.
Military ranges all around the country are some of the last bastions of various endangered species and habitats, simply because of the lack of access and other use. The idea that a large chunk of this wilderness would be obliterated to spare the residents of London some noise would not get past the planning system, however much it gets liberalised in the near future. Having grown up in west London, I have to say that I believe that urban noise is all part of the deal. City- and suburb-dwellers benefit from all sorts of conveniences, including access to services, culture, jobs, transport connections, etc. The downsides are usually noise, pollution and lack of space. City dwellers weigh these up and decide to stay put. Country folk value the peace, quiet and space above the convenience of city life. Why should it be OK to export noise and pollution onto them? Certainly the aircraft of today are far, far quieter (especially on takeoff) than those I grew up watching from my back garden some 30 years ago. The particularly influential bits of west London (Richmond et al) need to stop pretending that they are 'countryside in the city' and take a bit of pain for the good of the country. Which is what planning systems are supposed to inflict!