Biarritz - which ferry Bilbao or St.Malo and drive

Following heated discussion last weeked we have finally decided to visit Biarritz/Hossegor for our September 2013 caravan trip, hoping that it will still be warm enough for shorts/t-shirts/swimming at that time of year.  Living in Hampshire, Portsmouth is the obvious option for ferrys but should we go to Bilbao/Santander or to St.Malo and drive.  We only have two weeks total and don't want to spend 4-5 days travelling so Bilbao looks to make sense.  Any advise appreciated from the more experienced Euro Travellers :-)  How long would you expect the drive from St Malo to take (only one driver) ,  costs in tolls and diesel (average 23mpg whilst towing) etc.