Moving To Spain EU/EEA From Outside the EU - Importing Personal Belonging & Link Paperwork
Very helpful member
Now UK no longer in the SM and CU a 'third country' you need all the 'correct paper work' or will face paying customs, UK has a trade deal but 'local VAT is payable on the value of belongings'. For people moving to an EU country who can prove they have 'resided outside an EU/EEA country for 12 months' can transport your personal belongings to your new residence without Duty/VAT. You need a mountain of paper work to prove this & might be a little more complex if not traveling direct to Spain, 'VAT is due in the country where you are moving to'. Most removal companies should now be up to speed on the paperwork.
'This a very good page which will help, with 'link for paperwork', although it's for Finland 'the very same rules apply Spain.'
https://tulli.fi/en/private-persons/moving/moving-to-finland-from-a-non-eu-country