Hi there
I understand it is a bit like here in Ireland. (Not easy or cheap!!!) Spain has the dreaded VRT (Vehicle Registration Tax), I think this is at 7% of whatever they value the car at. (Just to get new number plates)
In theory, you are meant to inform the authorities if you are moving there and should register the car. My understanding is though that you can actually stretch it to a year on UK/IRL plates and on home insurance etc, once the car is taxed at home, then you MUST register it in Spain. this is what I am doing anyway!!!! (If you intend to do this, you must get a greencard from your insurance at home when you drive in Spain (most give 1 month a year no bother, some give more)
Hope this helps
Jason (11-14)