We do France for a fiver or 20 quid all the time...there are offers in the local paper quite often.
Since the ferry sails from Plymouth to Roscoff we take the car with us and head from Roscoff to a lovely town called Morlaix. They have a Saturday market where we stock up with cheeses, breads, roast meats, spices -you name it. Then we usually have a picnic lunch.
We then go to the supermarche and buy wine and local cider and more cheese and bread and things like All Bran Chocolate cereal and cheap t-shirts and kitchen items.
We normally stay the night in Roscoff or Morlaix, but have headed down the the Abers as well.
With the special deal you can stay max 24 hours in France. So a typical journey is this. Leave Plymouth at 11:00 pm Friday night, (sleep on the ferry -we usually get a cabin but that's 30 pounds extra), arrive 7 am ..stay the night and leave France at 9 am Sunday to return to Plymouth at 1 pm. The return trip is on their flagship the Pont Aven and it is nice nice nice.
There are also cruises to Santander, Spain but you only have about 6 hours on land. Our Uncle does this to buy cigs, plus it's three days with no one bothering him!