Must admit that i've never seen any kind of deal for Brittany ferries!!,but i'll take e'm very time,haveing done the Bilbao/Portsmouth(P+O),the Santander/Plymouth beats e'm hands down.
Have only ever gone one way,the worlds far to bigger place to over the same piece of ground twice on the same holiday.
What i have heard from friends who have been trying save a buck on cheap ferries,heaven help if you early or late,if you like to be organised fine,but on the Dover/Calais run i can't remember the last time i got on the boat i booked!!!,either going or comeing back,and with P+O it doesn't matter,cuz they run e'm like buses.