Once the unregistered car is at their docks and past the check in gate, go into the boat you dont need any permit at all just drive it into the boat (wait till 7pm when they are about to close the gate, I got there at 7.10pm was the last car in and without brakes for 30 years I almost hit their booth and later almost ran into a very late Mercedes V12 coupe and the security guides watching couldnt believe I stopped just in time! with handbrake only working as fluid in brake cylinders disappeared already and 2nd gear to slow it down) its private commercial property, park it behind the cars they tell you to and sleep all night at the sleeping deck single sofas chairs at the back (not the best sleeping conditions but cheap!) till the morning and load it up on your car trailer that you leave parked in their parking area and off you go but not after a few coffees!!