If you were able to get the ferry, would taking the Portsmouth to Bilbao ferry help cut out the long drive through France? You'd be quite close to the border and the Pyrenees from Bilbao so that might be do-able. I've not done this myself, but just looking at the map. It looks like these ferries offer you a choice of how to take a dog on with you, including in your car (which you've ruled out), shipboard kennels and shipboard pet cabins. Might be an option?
Cheers John. A cabin could be an option. He gets very stressed on the ferry. I’m going to look at what accommodation is available in the La Brenna region. It is very much a family holiday so I have to book something we’ll all like.