Check your UK insurance, they tend only to cover you as a visitor to a EU country (usually 90 days), once your registered as a resident or you’ve been away more than 90 days, your insurance can be invalid, as mos,t if not all UK insurance policies cover UK residents only.
Personally its not worth the hassle, your better off financially selling it in the UK and buying a cheap Spanish car.
Check out these for some ideas of price.
http://www.segundamano.es/
Most second hand cars are bought through free ads etc, not many garages on the standard of the UK offering decent second hand cars.
Your UK car will bring too much attention from the authorites and will be worthless in Spain.