Marble Arch or Charing Cross?..it depends who you ask or consult...
Wikipedia is quite often known for not giving the definitive answer shall we say...
Certainly Marble Arch is used consistently & from memory has always been the yardstick involved when it has come to weighing mileage for airport development certainly to the North of London...
Even black cabbies give different answers & if they don`t know...nobody does...