I've often been curious about this but never asked before. I'm learning at the flight centre at the moment and quite often fly out of EGPK on Ryanair.
I find controllers and staff at Prestwick to be some of the most helpful, friendly and accommodating people. So i've wondered before if it's possible for me to leave my car at PFC and see if a cheeky call to ops would give me a lift to the terminal. Not even sure that's possible though!
Anyway, chances are if someone with a car is hanging around at PFC, they'll probably give you a lift to the terminal.