During the process to find London's fourth airport, the old airfield at Thurleigh near Bedford was considered as a possibility. Distance from London = 105km.
LTN, LGW and STN are all over 50km from Central London, and try doing that journey in less than 2 hours on a bad day!
What distance is considered acceptable for an airport to be considered as belonging to a town/city?
subload PIK is shown as 54km from GLA city centre which is nearer than LTN, LGW or STN are to LON.