We have heard many horror stories... over the years, including different regions Guardia Civil interpreting the law to suit themselves. However, officially you can keep your car in Spain on UK plates for 6 months of the year. This means, that if you were to time it right, you could drive your car for the second half of the year in Spain and then roll straight into the first six months of the following year. However, keep in mind the following:
If you are living in Spain full time and have or will be taking "residencia" status, you lose the 6-month right altogether as you cannot be deemed a tourist.
*This is the most up-to-date information, but like so many laws and regulations in Spain, it can change in a heartbeat!