My wife and I took ferries between France and the UK twice in May last
year. If I recall we made reservations for ourselves and our car only a day
or so in advance. We took Brittany Ferries from Roscoff, Brittany, to
Portsmouth. From Dover to Calais, we took P&O Stena. Sorry I can't recall
the fare for either trip, though with a car it was not cheap.
However, I'm sure it's cheaper than taking the Shuttle, the Channel Tunnel rail service for vehicles. We took the Eurostar (the passenger train) between Paris and London, and it was quite expensive, about $150 round trip per person, if memory serves me.
Evan in DC