Depending on where you're staying France, is there a more convenient sailing time from Caen (or even Le Harve) instead of Cherbourg?
Or a later one from Cherbourg?
I've actually found Brittany Ferries incredibly helpful when having to change sailings once you get thru to the callcentre.
I missed a sailing from Caen (satnav taking us to the wrong place before we realised), and they got me on a Cherbourg sailing later that day with one phone call and an admin fee.
It's not like a plane where every seat is accounted for down to the 1p, the boats are bloody massive - if there is room, they'll generally get you on.
They aren't so flexible when you buy thru an agent though because those tickets are sold off cheaper.