I can't see much benefit to the Caen flights, unless you're a south Essex resident with a holiday home in Normandy, and even then, the whole point about having a holiday home there is that you can drive there easily with your own car from home. Brittany Ferries are really expensive but the Calais ferries are really cheap albeit a long but not unpleasant drive from Caen.
This route was tried for a while way back by Buzz from STN and didn't last long. Popular with French students but that's about it.
Rennes is much more likely to do well. Vibrant city where the Flybe brand is already well established.
I hope these niche flights work well for Flybe / Stobart...