Peter
If you are looking for an intresting route down look at the D roads.
I usually go back to Caen via Mayenne/Domfront/Flers great old style French roads.
If I was looking for an alternative route from Cherbourg, would consider dropping down to St Lo, then to Vire, onto Flers/Domfront/Mayenne/Conlie, this brings you out onto the N138 3 kilometers north of the le Mans ring road.
This is not the fastest route, but certainly more intresting than the N roads, and if you are in the spider you can pop past any slow traffic almost as easily as we can on the bikes.
Cheers Barry