Historically, Blackbushe was the alternate airport for London. Until Stanstead came along. The London Airport expansion plans are fraught with lots of vote losing ideas and the most practical one is to concentrate on Gatwick rather then Heathrow as there is more room to grow in Sussex. However whichever way you look, it is a loose-loose scenario short term. Luton has no room for growth I believe although the population to fill additional jobs is there. Farnborough has nicely tucked away the bizjet corner of the market, and East Midlands is a cargo hub shared to some extent with Manchester.
The optimum if unaffordable solution would be to think long term and build a brand new complex in the Shires linked to the high-speed rail and motorway complex north of the M25 based on an undeveloped but extant airfield such as Bassingbourne that has flat farmland surrounding it and developing towns and cities within commutable distance.