Those 7 trains all serve St Pancras and its Underground network of the Northern line, Victoria Line, Piccadilly Line, Hammersmith & City and the Metropolitan
Some of those trains also have direct links to the District Line, Jubilee line and the Bakerloo line via stations at Blackfriars, Elephant & Castle and London Bridge.
The truth is all of Londons airports are well connected once you hit the Underground.