Total agreement with CliveV, it really feels like your holiday has started when travelling on Brittany Ferries - friendly welcoming staff - great waiter service restaurant - and you arrive refreshed and close(ish) to your final destination of Southern Brittany.
We've used them to Roscoff, Cherbourg and Caen (Ouistreham) in the past - and assuming you need to pass Oxford on your way to the port - you will save about 1 hour of driving in the UK compared to Dover - and another 5 or so hours of driving in France to get you back to Caen in Normandy
So by going Brittany Ferries, you'll save a considerable amount of driving, maybe also gaining an extra day or two at your final destination. But compared to the bargain prices on the Dover - Calais route, you'll pay quite a lot more for the Brittany crossing.
If you want to work out the cost of the extra driving , use www.viamichelin.com to plan the route from Calais to say Caen - change the options to add a caravan and your car type - which I did for my C-Class and caravan - the predicted cost for the return Calais - Caen - Calais is an extra 700 km of driving and road tolls and fuel add an extra Euro 150 (