Booking in advance saves on messing about at termial and having to get out your van and go buy a ticket - plus its cheaper in advance.
We went with P&O and paid about £45 return.
A benefit of P&O is they will prob bump you onto an earlier ferry F.O.C (if it suits you - if theres room!).
We did, we were offered and took an earlier trip both ways - brilliant A:. If you dont ask for it you dont get charged any extra - if you ask there is an admin fee .
Only works within a couple of hours of your sailing.
worth a thought during planning stage
They have Licence plate recognition too - excellent stuff. I drove up to the window and the French lady greeted me with a smile and addressed me by name.