Writing on a Guardian blog, Andy Beckett reminds us that Edward Heath's government fully intended to build a similar scheme at Foulness (or Maplin as it more euphoniously came to be called) in Essex:
It would have multiple runways and new transport links to London, to relieve the already-notorious noise and congestion at Heathrow and help regenerate the poorer eastern side of the capital. Heath was very keen, and the plan quickly acquired faintly sci-fi features: "a brand-new jet city" to be built nearby, "tracked hovercraft" to whisk airport users to London. Speculators bought up whole streets around the planned London terminus near King's Cross.
In 1973, construction started on the grey-brown Essex mudflats. A gravel "trial bank" 300 metres long was erected, to see if an airport could withstand the North Sea storms and deep estuary quicksands.
Riding the rising tide of early 70s environmentalism and political militancy, local opponents of the project organised themselves flamboyantly as the Defenders of Essex, and seduced visiting reporters with picturesque arguments such as the damage likely to be done by the airport to Southend's cockle pickers. Meanwhile, in Westminster, an unholy alliance of Labour MPs and austere Thatcherites-in-the-making such as Norman Tebbit questioned the rising cost of the scheme to taxpayers.In those days the easiest way to get a round of applause on Any Questions? (the panel usually seemed to consist of Richard Marsh, Arianna Stassinopoulos, Michael Clayton - "editor of Horse & Hound" - and Russell Braddon) was to call for prestige projects like this airport, Concorde and the Channel tunnel to be abandoned. And eventually the airport and the tunnel were abandoned.
It turns out, if you read an excellent paper published by the House of Commons Library - Aviation: proposals for an airport in the Thames estuary, 1945-2011 - that Maplin and Boris Island are just two of many schemes for an airport on the Kent or Essex mudflats.
You may enjoy the Pathetic Motorways entry for the unbuilt M13 too.