Hi SLF3,
Your point of moving "London's main international airport" is an interesting one. IMHO, Heathrow is a victim of it's own success and location. Being located just 10 miles from London makes transport to and from the city easier (Train infrastructure - now there is something that is a MAJOR joke!). The vast passenger numbers that pass through it does cause the airport to creak and groan under the pressure. And it survives....just.
However, locations that are going to be as accessible to London are almost non-existant. The Cliffe idea was a pipe dream, considering not only the much publisied enviromental impact, but impacts on other airports traffic flows, as well as airspace implications with our very close european neighbours. Stanstead expansion is to go ahead, but once again Heathrow "Historical" attractiveness to those wanting to travel to London combined with Stanstead's proximity (or lack there of) to the city will make it less attractive to carriers and passengers alike.