See http://www.bbc.co.uk/news/business-29026484 .
For the moment, they seem to be rejecting plans for an "all new" London airport in the Thames estuary as Boris Johnson would like. It would certainly ruin bird life in the area.
The argument is that in the future existing airports,even with more runways, will not be big enough. Is that true? The arguments for a "Hong Kong" like solution for the longer term makes some sense. I doubt politicians are brave enough to make such long-term decisions. The environmental lobby will be against a new airport: in the end there may be no choice. Heathrow is a daft airport with poor access and facilities. If starting today, you would not build an airport there.