I just wanted to stick this up here in case someone finds it useful. National Ferry Fortnight is running from 2-16 March, and there are various offers and discounts on the go.
We booked with Brittany Ferries to Caen, and got free travel for the DS, and a cabin for free (just on day crossings, though, and not the biggest posh cabins either. I checked. Still: free! And somewhere to have a shower and a rest after an early start. Not a holiday if you don't start out before dawn). I think quite a few companies are doing discounts, free child travel, etc, and there are offers for travel to the Netherlands and the Channel Islands as well as France.