The UK is no longer in the EU and the process and costs to register a UK car in Spain are a lot more than they were before Brexit.
This is because UK cars now are now treated as vehicles from third countries and must clear customs, have import duty and VAT paid, or get a waiver if your car qualifies for customs exemption.
The process you follow will depend on your situation, as will the cost. You’ll need to budget for costs of around €1,500 with taxes as applicable on top.