In the Competition Commission’s Market Investigation of BAA an eagle eyed colleague spotted yet another blunder in the world of aviation expansion - there's not enough space.
27. Airspace is a finite resource. Safety is the absolute priority for the management of air traffic flows through a given volume of airspace. This is achieved through the application of, internationally agreed, horizontal, and vertical separation criteria that are applied to all aircraft operating within defined volumes of controlled airspace. In the southeast of England where there are multiple airports with high traffic volumes of aircraft with different performance characteristics on a wide variety of departure and arrival routes, there is a limit to the amount of traffic that can be accommodated safely at any one time.
All credits to 'B'.