As I understand it, you can keep an English plated car in Spain for up to 6 months in any one calendar year. Make sure your MOT doesn't expire in that period, or you're completely snookered, as you have to get it back to the UK for an MOT. If you decide to keep it in Spain and put it on Spanish plates, you can't then take it back to the UK ever (unless you become a Spanish resident yourself) as it is illegal to drive a foreign plated car in your home country.
Regards,
Adrian