I agree Brittany Ferries charge ridiculously high prices. My parents have a house in Brittany so the preferred route is Plymouth to Roscoff. It really is the most convenient. My parents are currently in France, so myself and three children aged under 7 are going over as footpassengers (from Plymouth, to be met in Roscoff) on Friday 28th May, returning Thursday 3rd June. I have booked a cabin for an extra 20 each way. Total price for the return journey is 205.10, and that includes the Guest discount. Two hundred pounds as foot passengers! Crazy price.
