From UK it is still approx 1200 miles so a good 24 hour drive in total, 2 overnight stops would be advisable.
depends if you have a preference of route, take a look at www.viamichelin.com for suggestions.
if you do the Paris Peripherque and straight down, take a look around Millau and the Gorges du Tarn, then towards Perpignan and down the Med coast.
if you prefer to miss Paris, head toward Rouen then down west coast towards Bordeaux and Biarritz, look at Pamplona or San Sebastián in Northern Spain.
be very careful of the various scams that happen to UK reg cars in Europe, keep valuables safe and keep an eye on your car at all times.
lots of info on the Driving in Spain thread on the Discussion Boards.