Prices are very reasonable for taking your bike to France with Brittany Ferries, generally any return is around 60 quid. I'm researching a solo ride for sept, ferry into one port and cycle to another.
Rough numbers so far:
caen to st malo 125 miles
caen to cherbourg 93 miles
le harve to st malo 170
le harve to cherbourg 140
Any of those would represent a good 2 days cycling, but cycling Caen to St Malo currently looks favourite. Need to research cycle routes now and then look for cycle friendly hostels or bunkhouses. :o)
If any friendly STW people are in Portsmouth, would you maybe consider letting me leave my car in your drive for 3/4 nights? :o)