Done a bit of homework...Option 1
All St Malo and Caen boats are 6+ hour crossings. Weekdays are a bit cheaper. The pick of these looks like Caen: Depart Tuesday 7 July 8:15 (£119) return Tuesday 14 16:30 (£119) Option 2
Cherbourg fast boats work out £30 extra each way. No price difference for weekday verses weekend. The pick looks like: Depart Monday 6 July 9:00 (£149) return Sunday 12 July 20:45 (£149) Option 3
Le Havre prices the same as Cherbourg for fast boats, but depart times are very early AM.Sum up
I've assumed there will be several hours driving after arriving in France, so have picked early boats. Plenty of mix-and-match options if we want to depart and return on different routes, with different trip lengths, etc., but then it all gets a bit complicated for little me.
Option 2 is my preference.