I used pre-booked parking at London Luton Airport, the process was simple. When you arrive at the airport and drive up to car park barrier a machine prints out a ticket (which you retain for later use on the way out of the car park at the end of your trip) - you then park up and leave the car in a large car park within walking distance of the terminal. On return from my trip I you recover your car and drive to the barrier, inserted the ticket and may find the machine asks you to pay a charge - in my case a heart attacking £108!! (as mentioned previously this was pre-payed). However, there is a call button on the barrier, a member of staff will chat to you on the radio and asked a few security questions, before they raise the barrier and let you escape, if you haven't pre payed then...? Overall this was a good parking option for those on a budget who don't need valet parking.