Closest tube stations
From Gatwick Airport
From Heathrow Airport
From Stansted Airport
From City Airport

Train: Heathrow Express to Paddington Station, from there by Tube (Circle Line or Hammersmith & City Line to Moorgate or Liverpool Street)
Tube: Picadilly Line to King's Cross St. Pancras from there Circle Line or Hammersmith & City Line to Moorgate or Liverpool Street
See:  www.baa.co.uk/main/airports/heathrow