It is my impression that on the BF Portsmouth - Santander route you have to book a cabin as it is an overnight crossing. I tried earlier this year to book a crossing with either bicycle or motorbike but couldn't complete without selecting a cabin which was an horrendous price. You couldn't just select unreserved seat then crash down in a corner on your thermarest like ferries in South East Asia. Also they don't take bicycles as there is no bicycle option under choice of vehicle. I emailed them about this and they told me to call them direct to arrange booking which sounded a bit sus, which it was, as when I did I was told by their sales operator no bicycles on this service no exception. It might be that you can take bicycles on shorter BF crossings i.e. between the UK and Caen, Cherbourg, Dieppe, etc but not to and from Santander/Bilbao. But I am sure some one will be along shortly to prove me wrong that they have indeed taken their bicycle on the Santander route to either Portsmouth or Plymouth this year. You can take a motorbike but not a bicycle. Off season the service it cut right back. How about taking a local train half way either on the way out or back so you don't quite so far to cycle? If you are a person of leisure and don't have time constraints, why not make your cycle tour France a round trip? No planes trains boats or automobiles?
"Zat is ze reel prowoking qwestion Mr Paxman." - Peer Steinbruck, German Finance Minister 31/03/2009.