Think you will find the ferries go to Normandy,that is the ports of Cherbourg and St Malo. St Malo being the closest to Brittany.

Patka

There may be a fast ferry/catamaran, but it will be to Normandy not Brittany. Depending on where you are travelling from you will probably find that the overnight service with Brittany Ferries from Portsmouth to St Malo will allow you to reach accommodation in Brittany during the afternoon. But it is an expensive crossing and you will have to book up very soon to be sure of a place!

Yes, there are roads from Normandy to Brittany, but they are by no means fast!! You could try planning an alternative route from a Normandy ferry port, eg Le Havre, Caen, Cherbourg etc to your destination with http://viamichelin.fr.

Condor run fast ferries from Weymouth and Poole to St Malo with a stop in the Channel Islands en route.