Obviously you can't get to London directly by ferry, but it's only a few hours from the ferry terminal to London. Regular service is available between France and Newhave, Dover, Folkestone, Harwich and Ramsgate. If you wanted to drive, the Eurotunnel connects Folkstone and Calasis, France. There's also Irish ferries to France, departing from Rosslare to Cherbourg or Roscoff.