"It is anywhere that a member of the public is not allowed without a boarding card, the car won't even get close to that."
But it's more nuanced than that - isn't it. I have driven my car (or more pedantically, a rental car which I had a contract for) to an aircraft at a major UK airport. I did not have a boarding pass. I'm not going to spell out the full circumstances, as most on this forum will understand how and why that was possible/permitted.
It has come as a major shock to me (based on the other insurance question thread) that I probably didn't have insurance cover while I was airside (even if we haven't been able to fully define what airside is).