It's worth trying to speak to the ferry company as some carry 'wedges' to enable low slung cars on and off.
I went on a ferry with an F40 that failed the 'coke can' test. This is where you couldn't roll a coke can under it as it was so low. It was slow loading it on - but we did it.
I generally take the tunnel - 30 minutes crossing, stay with the car, no drunken tax free shoppers, more driving at the other end.
