Historical version 1 of Category Airports (view current version)
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:-
This is version 1
(as of 2003-06-15 14:08:42).
View current version.