Why should the Queen enjoy unrestricted travel on government airplanes?

Fair enough; it's her air force.

(ObSF: Omega by Stewart Farrar. At one point the entire royal family disappears from an RAF base supposedly on total lockdown. It turns out the security breach came from the king walking into a hanger and telling the poor bloke on duty, "Prepare a helicopter; I'm going out." It is the Royal Air Force, so...)