I know this is an old post but having previously made the same mistake I wanted to advise you what I was told to do in busy summer periods! If you want to travel a-n-y time between May-August, pre-book it at least 6 weeks in advance. If it's a Brittany Ferries crossing (usually mini-cruise type) then allow even longer! With
LD Lines being a much cheap ferry company you must expect that it will be extremely busy also. Let's hope their fast craft comes into operation this summer as it made life so much easier!