We aren't quite as quick of the mark as you, but we used to stop in Ouistreham (Caen port) for a meal on Monday night, after a hard days shopping, and catch the 11 pm boat. We then discovered it was cheaper to stay in a hotel compared to the cost of a cabin/peak rate for crossing. Going back Monday night meant work on tuesday was difficult, so you might as well have the day off.
Hotel rooms range from about £25 a night for the local stuff to about £55 for the Mercure.
You then catch the 9 am ferry on Tuesday