Found it, thank you rayo :)  It was the 'economie' bit that helped.

http://www.brittany-ferries.co.uk/economie

"We open for sale shortly. The first sailing will be on Tuesday 25th March from Portsmouth to Le Havre.

Join our mailing list to be amongst the first to know when we go on sale."

It looks like the crossings will take longer than they do at present and I really would recommend having a cabin for the crossing.  More than 24 hours with just a reclining seat will leave you cream crackered before you even contemplate the onward drive.

PS I have asked them to clarify the Santander to Portsmouth sailing times as it looks like it leaves at quarter past two on a Sunday afternoon but doesn't arrive until eight thirty on the following Tuesday as it goes via Le Havre.