Also, don't forget that you must carry a minimum of 1ltr of fuel in the boot.
My experience from last year:-
I bought 2x 5ltr plastic 'cans', because a 1970's british sports car would not make it without a fill up.
I was first car across the line at Newhaven and, the ffirst car in for a search at customs! What the ***k can you carry in a 35yro Spitfire!
I was asked:-
Do I have any knives, guns, explosives? Only cutlery for knives.
Do I have any spare fuel in the car? please open the boot. 2x 5ltr cans.
Can you put the fuel in the main tank? No. I just filled up.
The ferry company will not allow you those on board, we'll give a you receipt but hand them over please.
Advice, take an empty fuel can on the ferry, I had 10 ltrs confiscated last year.
So fill the can up over in France.
BTW, on my return when claiming my fuel, one can was EMPTY!!! OK, the Port paid me a fiver for their mistake. But take head.