Website URL: http://www.brittany-ferries.co.uk/faq/travel-information/do-i-need-a-passport-for-a-daytrip

Do I need a passport for a daytrip?

Yes, every member of the party requires a valid passport, and a visa if necessary (visas are not required for France and Spain by EU Nationals). It is the passenger's responsibility to establish his or her own individual requirements with the country of entry and re-entry.