Here's a wacky suggestion - or is it?

There are already some runways close to LHR. One is at Wisley (12 miles way, disused but intact) and the other at RAF Northolt (5 miles way and quite under used). Build an ultra high speed HEX type train shuttle linking them to LHR main and there are your extra runways. Clearly not as convenient as a wholly new runway at the main site, but would not involve major community demolition. The major infrastructure is the rail link - put it underground and you solve even more problems.

I think it has been studied before to say put UK domestics out to RAF Northolt (IIRC it could handle up to a 319 / 737) but I'm not sure if that was before the A40 was realigned. The key to make it all function as one airport is the speed of a shuttle transfer. 10-15 minutes xfer max and pax would feel it is one airport and not be too inconvenienced. And then there is Biggin Hill, Hatfield, White Waltham etc but they are further away.

I also wonder about the suitability of say RAF Odiham in the future to relieve LHR a bit?. It was also earmarked for closure, has a reasonable runway, about 2 miles from the M3 J5 and close to the main rail line into London. Perfect - the Germans have converted some of the former USAF bases into commercial airports so why is this not considered in the South-East of the UK where completely new runways seem impossible to build?

LHR is now really in the wrong place for further expansion - probably one of the few major airports in the world where the main approach paths are right over a major city. Post 9/11 and with noise pollution / air quality becoming more of an issue every year, something has to give and more radical thoughts applied.

I used to work in the rail sector and it was often a subject of what a missed opportunity when the M25 was built not to have added a high speed rail "ring" in between the C/W and A/CW carriageways - even just the M23-M4 quadrant. Like you see in between the lanes of the A4 from Schipol into Amsterdam. Think what possibilities of an integrated airport strategy for LHR / LGW / STN that would have provided if you could link all them together with a high speed interconnect. Again, if you made this high-speed enough you could function as (almost) a single airport.

Still the good old M25 was built in the Thatcher years and rail was history and the car was king.

Just my thoughts at this late hour.