Lots of fuel and time for sure as it is a long way across. Not many tolls in that part of France though. Of course your nearest port is Roscoff served from Plymouth by Brittany Ferries who certainly aren't cheap, You can also sail with them from Portsmouth or Poole to St Malo, Cherbourg or Caen which cut the driving down. You can EARN Avios points by booking with direct ferries but 99% sure you can't redeem them in anyway to pay.