Generally, if you want to receive goods from another EU community member country without paying VAT in the originating country, you will need to be registered to do so in the EU Taxation and Customs database. If you are not registered there, any supplier should charge you VAT at their rate. If they charge you with VAT included, then you can be treated as a general consumer and therefore do not need to be registered in Spain, nor in the UK. Speak to your accountant though. And to your supplier, it’s possible that their policy is to only deal ‘trade’ with VAT registered companies, if this is the case, the most logical thing would be to register at home.