The Open Guide to London: the free London guide - Differences between Version 3 and Version 2 of Category Airports
Contents are identical
London officially has 6 aiports. Although only 2 of then can be considered to be really in london and even then it's pushing it for Heathrow. It would seem the main criteria is to be reachable from a London Rail Terminus in about an hour or less.
The 6 are:-
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