Only experience is from back in the early 90s when I was working in a site office airside at LGW and had to drive there due to distance from any staff car park (it would have been a convoluted half hour walk)
My vehicle insurance did not cover me anywhere airside & insurer would not countenance extension of cover except at an absolutely extortionate rate (in the tens of thousands of pounds if I remember correctly); probably due to ignorance of the demarcation between airside roads & aircraft manouevring areas.
My employer (BAA) however did cover my vehicle (icluding £50Million Third Party liability) but it did have to have magnetic signs & an orange flashing light, again, happily supplied by them.
I was not allowed to go onto manouevring areas because of lack of an R/T fit but did have to cross an active taxiway by means of the airside road.
This was nearly 20 years ago and I believe that post Twin Towers etc. the rules have been considerably tightened.