the airspace constraints that have been mentioned by some other posters will be difficult to overcome without a significant change to some of the widely held principles and/or freedoms claimed by some people. Maybe a more significant problem is the limited gains in movements that a third runway would be likely to provide - something that in my experience is not understood by the layperson.
Would there not have to be a cut in slots elsewhere ?
How does this work ?
I assumed that when BAA owned LHR LGW and STN there was an element of slot juggling, now all the airports are all owned by different operators airlines at say LGW who do not have service or partnership with LHR are not going to give up slots are they?