The demotion of Justine Greening is being taken as evidence that the Cameron-Osborne buddy act are now committed to a third runway at Heathrow, something that the astute self-publicist Michael Ryan (now champion of Stansted) has conceded. That other noted self-publicist, Boris Johnson, has stuck his ample oar in, favouring as he does "Boris Island", a vanity project on a par with his hero Churchill's disastrous Dardanelles plan, though hopefully with less scope for casualties.
The idea of an airport in the Thames Estuary was first mooted back in the 1940s. This is not some brilliant new idea that Boris has come up with, any more than the "Boris Bike" scheme was. The reason it has never got off the ground is not because of worries about birds nests being trampled but because of more fundamental issues, namely: the increased likelihood of bird-strikes and fog in an estuary; the remoteness of the site from London (the idea’s reappearance owes much to the presumed handiness for the High-Speed 1 line at Ebbsfleet); and the fact that users would generally prefer to go via Heathrow, or any other airport nearer Central London.
The problem with airport capacity is not primarily that we are travelling more (the big increase in domestic use happened decades ago), but that the rest of the world is doing more travelling, most of which has to connect via a small number of international hub airports. London is one of the world's primary hubs, offering a connection between the western and eastern hemispheres, as well links between Africa and Europe, and polar routes to the Far East. It is vulnerable to competition from Paris, Amsterdam and Frankfurt, but it is better positioned for intercontinental travel and, Britain being an island, there will always be significant domestic traffic to build on.
The ideal location for London Airport is to the south, giving easy access west, south and east, with only the smaller volume of traffic to the north flying over the capital itself. Today, the bulk of traffic flies over the city, most of it along the Thames. London's first major airport was to the south at Croydon. The decision to relocate to the west was a mixture of poor planning (expansion land around Croydon was built over in the 1930s) and opportunism (a large military runway had been built at Heathrow in 1944). An eastern position would be next best, however that would be costly because of the need for additional transport infrastructure, and would only be viable if Heathrow and probably one other airport closed altogether (i.e. 3 runways replaced by 4). There isn't a lack of capacity across the London airport system as a whole, just a bottleneck at Heathrow. Boris Island's 4 runways would actually deliver over-capacity, which is why the economics don't stack up.
The problem with the London area is the poor rail connections between the airports, which makes it difficult to disperse connecting traffic to other runways with available capacity, produing the Heathrow bottleneck. It's worth remembering that the London area has 7 runways spread over 6 airports: Heathrow (two), Gatwick, Luton, Stansted, Southend, and City; and that's without counting the currently non-commercial runways at Northolt and Biggin Hill. The idea of linking Heathrow and Gatwick by a high-speed rail link, creating a single virtual "Heathwick", has been mooted, however this is less popular with the airlines. They'd rather expand Heathrow and keep low-cost flights running out of Gatwick.
In fact, there's an argument to increase the virtual airport to include Luton (Heathwickton, perhaps). Not only does this increase capacity (assuming low-cost flights migrate to Stansted), but it means that further runway growth can potentially occur in two locations before Heathrow need be considered. This becomes even more compelling if the HS2 link is diverted to run via Luton airport, thus providing fast access to Birmingham as well as London. (There is also a case to be made for extending Crossrail, which is due to terminate at Shenfield in Essex, to Stansted in the longer-term.)
Without a high-speed link between Heathrow and Gatwick, London airport transfers will continue to be a major hassle. There are fast connections into Paddington and Victoria, but you need to sit on the Circle line for half an hour in between and cart your luggage through two stations. Crossrail will extend the high-speed route from Paddington to Tottenham Court Road and Liverpool Street, but it won’t help transfers via Gatwick or Luton. There are plans for a Crossrail 2, which will provide a spur from Tottenham Court Rd via Victoria to Chelsea (and possibly Clapham Junction), but that is years away and the economic case looks weak. This might get the Heathrow-to-Gatwick transfer time down to 1 hour. A high-speed rail link close to the M25 would be a lot easier (and cheaper) to build than more routes in Central London, which would have to be largely tunnelled, and it offers the prospect of transfer times closer to 20 minutes.
Boris Island would leave cross-London transfer times little better than they are today, even with an extension of Crossrail to Kent. This would argue for the minimisation of transfers, which means building the maximum 4 runway capacity from day one. The green light for this folly would therefore be the death knell for Heathrow. While a few nimbys in Putney and Twickenham would be happy enough with that, the impact on the local economy of West London would be traumatic, and arguably the entire Thames Valley would suffer. Should businesses consequently relocate from Maidenhead to Maidstone, this would further leech opportunities away from the Midlands and the West Country. The UK would become even more unbalanced.
The commission of inquiry into the Dardanelles Campaign failure reported in 1919. This sums it up neatly: "It concluded that the expedition was poorly planned and executed and that difficulties had been underestimated, problems which were exacerbated by supply shortages and by personality clashes and procrastination at high levels". The solution to London's airport conundrum is not more runways, but better railways. Now we just need to convince Boris that it was his idea all along.