Alongside the Brittany Ferries routes to France and Spain, Portsmouth also has a ferry to the Channel Islands: Guernsey, run by Condor Ferries, now part of the Brittany Ferries group.
Routes from Portsmouth
Channel Islands or France/Spain?
Worth deciding on early, since these are genuinely different trips. The Channel Islands routes are shorter, land you on a small island with no long drive at the other end, and suit a shorter break. The Brittany Ferries routes to Normandy, Brittany or Spain are longer crossings that open up a whole country by road once you land.
See the operator itself on our Condor Ferries page, or compare every route from Portsmouth on the Portsmouth ferries overview.