I don't see a runway that is lined up with Windsor Castle getting the go ahead, and nor do I think it should. The planners chose a bad location when they decided on a new London airport to replace Croydon - it was good for transport links but too limited for physical expansion and growth of traffic. This extension is not going to work well - what's the point in increasing the runway capacity by 50% at a curfewed airport if you can expand LGW by 100% with no curfew?
My biggest doubt over LHR is a simple safety one - I don't think it's a great idea having so many arrivals and departures over such a congested area. BA038 was very lucky with its energy profile that it came down in a clear area. Then there are the departures - remember the STN 747 cargo accident? I don't think increasing an airport buried in conurbation is sensible - airports should generally have relatively clear undershoots if possible, and at LGW and STN it is possible.