On arrival, Spain applies Franquicia por traslado de residencia (AEAT) under EU customs framework (jurisdiction-specific). Import VAT in Spain is 21%, and relief is what keeps your household goods out of that charge. Shipments normally arrive through Valencia or Barcelona.
This page covers what actually changes on the United Kingdom → Spain route: what you will pay, how long the shipment is at sea, the paperwork Spain customs expects from a departure in United Kingdom, and what to do about pets, a vehicle and somewhere to live when you land.
Departures from United Kingdom normally load at Felixstowe, Southampton, London Gateway. Since leaving the EU customs union, UK household-goods exports require an export declaration filed to HMRC. Removal companies file this routinely for personal effects. In-home surveys are standard in the UK and usually available within a week or two of enquiry.