Whatman
We did the journey via Eurotunnel 2 years ago and plan to do it again this April.
from SE UK it is approx 1200 miles, shortest route, which is probably around Paris to Clermont Ferrand and down towards Perpignan crossing the fabulous Millau Bridge over the Tarn then down the A7 all away down the coast and costs will depend on
Diesel or petrol car
How many stopovers do you want, possible to do one but not recommended
Do you want to avoid tolls and use slower more picturesque routes
A good starting point is www.viamichelin.com or Bing maps you can choose your own journey
Make sure you have good printed maps as well as Europe cover Tom Tom, European breakdown cover and study EU driving rules and regs, avoid valuables on show and stay alert, scammers love targetting UK reg cars
Personally we will take a week or so, turn right at Calais towards Rouen past Le Mans and down to Bordeaux crossing the Pyrenees somewhere and heading perhaps via Pamplona and Rioja country skirting Madrid but stopping in Cuenca before hitting Murcia then down to the coast . We will try and visit some more interesting places and stay with friends in France but it is very much a personal choice.
Let me know if you need more details such as good stopover towns depending on your likes.
Do you want to do it quickly, or do you want to make a fun trip