Most ferry companies accept dogs with a pet passport. For a full list see here:
http://www.dogsaway.co.uk/main/routes.htm
Most operators don't advertise rates, or give much other relevant information, on their website. SpeedFerries (Dover-Boulogne) is a welcome exception:
http://www.speedferries.com/speedPets.php
(Your dog has to remain in the vehicle. I believe that this is common to all ferry companies. However, the SpeedFerries crossing is only 50 minutes. Get to Dover early, take your dog for along walk, and he'll sleep through the journey).
Chris