I agree tunnel is the best way with a dog/dogs, you can buy a harness which clips into the cars seat belt at most pet stores in UK, if you don't want your pets in a cage.
The French authorities also require you to have two breathilizers in your car, as well as the high vis vests and triangles, I think they sell the 'kits' at the tunnel terminal in UK.
When we have driven to and from UK we have made two to three stops overnight in France, it is less tiring after a very long drive, plus you can enjoy the food and wine before a good nights sleep.
When you are ready to travel back from Spain to UK your dogs will have to have a worming tablet Drontalgiven by a Spanish or French vet, before they can enter UK, i think it has to be given not less than 24 hours, and not over four days before travel, if I am wrong on the timing I am sure someone will correct this, but it should be on the defra site.
The tablet must be given by the vet, and the time, date and surgery stamp on it, they are very strict on that.
Over all we really enjoyed having our little travelling companion with us, and he is great in the car.