When we were putting our aircraft product together we decided to take the line that the airside cover in respect of cars would operate at the home base of the aircraft anywhere other than where the vehicle would require cover to meet the road traffic law. This avoids having to worry about defining the limits of the airfield or airside cover as it effectively ensures the client is covered one way or another.
Out of interest, a motor vehicle only airside cover might protect against potential liabilities arising out of the use of a vehicle but an aircraft owner can face other liabilities (e.g. if they knock over a ladder in a hangar and it prangs another owner’s plane). It is worth ensuring that you are also covered for such exposure while airside.
Cheers
Bob