I think its an excellent idea. They did it in Hong Kong. Airports should be kept away from built up areas at all costs. Not only noise but safety in general. Most accidents happen during take-off landing, there has already been a Trident into Slough a 777 just inside the boundary. Also most of the recent accidents that spring to mind seem to happen close to the airport Madrid......blah blah (even UK accidents Kegworth, Manchester).
An airport out in the Estuary (24 hour operation) with fast rail links into Central London, solves all the issues with regard to expansion at LHR-it may be costly but it would be worth it.