The internationally recognised standard for location of an airport in relation to its city centre is 50km. Only City (0km), Heathrow (25km), Biggin Hill (29km) and Gatwick (45km) Airports fall into this category. Luton (54km), Stansted (63km), Southend (67km) & Oxford (95) Airports are outside the standard. But then transport accessibility is also a factor. Using the quickest transport methods, Gatwick and Luton are 30 mins from London. Stansted is 47 mins. Oxford, Biggin Hill, and Southend are 55 mins. Heathrow a short 14 mins, before the advent of fast trains direct from Paddington it was 35 mins by taxi, 55mins by Piccadilly Line. This is why I prefer using Eurostar when travelling to/from Europe, less hassle getting to and from airports, both ends.