France in September ????

Arrivakids

I have read some Posts and a few people seem to go to France in September. We have just come back ourselves and had  a wonderful time..We normally do Plymouth to Roscoff but Maybe  looking at a  Shorter Crossing  and perhaps Normandy.. We have a Car Caravan
and Dog.. We have used Brittany Ferries Twice Now which we find very good but not the Cheapest.. Any Advise on a  Cheaper Alternative for September and what should we expect Weather Wise..