Hi Sian
We have frequently used the Plymouth to Roscoff route. We usually travelled overnight both ways. We had cabins on the boat, we don't recommend the reclining seats as although in darkness the rooms can be rather noisy with kids, etc and the chair are not very comfy for sleeping.
Travelling overnight and having cabins meant that we were fresh and ready for the drive at both ends and off got off to an early start, ie, about 6am. Plymouth and Roscoff are both easily accessible ports. Be warned, however, petrol stations are in short supply at night once one leaves the M5, if travelling that way. Petrol stations are, also, in short supply in France if arriving early, late or on Sundays.
Roscoff is a very easy port to arrive at as the port is away from town and there is easy access onto major road systems. Roscoff is excellent for all parts of Brittany. I would strongly recommend Camping Du Letty in Benodet - it is our favourite site in all the world and we have returned numerous times. Sorry to disagree with previous post but it is FAR SUPERIOR Camping Point St Gilles. You can read my review of Du Letty on UK Campsite here:
http://www.ukcampsite.co.uk/sites/reviews.asp?revid=5461
Camping Du Letty's own website is:
http://www.campingduletty.com/jeucadregb.html
Benodet is a coastal resort just beyond Quimper. Both Benodet and Quimper are easy drive of under a couple of hours from Roscoff. The AA route planner service on the AA website provides free and excellent route plans from Roscoff to Du Letty and a number of campsites and towns in Brittany.
A final word of warning the Plymouth-Roscoff boat is very popular and gets very booked up during the peak Summer holidays. Early booking is recommended to get the sailing you want. We booked the ferry through Camping and Caravanning Carefree Travel service they always gave good deals. Don't book the camp site through them its cheaper to book direct in my experience.
I hope this is helpful. Even writing this brings back MANY happy memories. Unfortunately redundancy limited us to the UK this year but at least it's good to share one's experience.
Best wishes
Tim.