Would suggest that you buy the Michelin Local Series, numbers 309,316,324 for the purposes of your trip. If you try to cover it using just one map, the chances are that it will be much less detailed and that you will need to resort to busier roads for much of the trip, making your cycling experience much less pleasurable. You should be able to get these from any large book shop or alternatively order them from a map specialist such as Stanfords.