You will need to notify the DVLA, first. You can do this by sending the permanent export form from your car's V5C logbook. Once you have done this you will be able to register the car in Spain with the remaining part of the logbook.

I would suggest using a gestor (who specialise in Spanish administrative tasks such as this). You can find a full list of registered gestores, here www.gov.uk/government/publications/spain-list-of-l...n