Air Force One usually isn't on the ground for long enough to need security - the aircraft only takes on that call sign when the President of the United States is on-board.
The same applies for 'Marine One', the helicopter that transports Presbo.
I thought the SS was responsible for providing security of the aircraft when on the ground?
As an aside, has the British government allowed an exception for security personnel protecting the President to carry firearms? They're usually armed in the U.S.