We took our [now just] 2 year old Cavachon away with us to France at the end of July for a week. Brittany Ferries do some dog-friendly cabins, and the whole ferry experience was absolutely fantastic. She was good as gold throughout the holiday, and I think just loved being with us and the walks. The pet passport thing was easier than I'd expected, too - there's a good website that shows on a map vets that British people have recommended for the pre-return check-up/worm treatment and who you can communicate with with only a 'petit peu' of French.
Want to do it again next year!