I love travelling with Brittany Ferries and have done numerous trips with them over the years. The majority of these have been day trips either with a car (more recently), or on foot. I also did a package trip to Santander once, and then hired a car which I loved.
As much as I like the day trips (typically to Caen or St Malo and back), unfortunately you have to rush around like a mad thing to see places and always seems a shame to get back on the ferry at the end of the day, rather than extend the trip. So why not extend the trip I hear you ask? Well it just seems to cost so much more, e