The easiest way is to take the Portsmouth to Saint Malo ferry. The drive from St Malo to Guilliers is just over the hour through much pleasant countryside.
Ferries are also available to Caen, Cherbourg and Roscoff but they are longer drives, although the ferry crossings times are shorter and generally cheaper.
Crossing via the Channel Tunnel means a drive of about 400 miles in France, though the motorways are very good. From Caen onwards to Guilliers there are no tolls to pay.
There is an airport at Dinard with flights from Birmingham, London or Leeds.
Dinard is about 45 minutes drive from Guilliers.
Rennes, about an hour or so away also has an airport.