Have you booked your ferry - or do you intend to just book when you arrive? It can be quite a bit cheaper to book in advance, especially on the late night or early morning sailings.
We've stayed in Dover (found a parking area somewhere where we could stay with the car and caravan, but it's so long ago I can't remember where) but more recently have just turned up and taken whatever ferry we were offered - making an early start in France after sleeping at the port, or a service area, the next morning!