In Ireland we have disccusions about importing receivers from GB. People often ask about "Duty", but by far the most common tax is VAT. The exporter in Spain should deduct the local Spanish VAT from the price. Then that base price plus the cost of postage and packing will have 20% VAT applied when it comes into GB. Whoever collects that charge e.g. Royal Mail, will also have their own handling fee.
But some exporters in GB do all this at the point of despatch in agreement with the EU. And then there is nothing for the customer in Ireland to pay at the point of delivery. It does not make it any cheaper, except the delivery handling fee won't apply. I don't know if exporters in Spain are doing this for parcels into GB.
Customs Duty probably won't be an issue as many products will be 0% rated, but it is up to the importer to do the research. Tariff codes are a whole world of wonder. If there is Duty, it will also be put into the calculation for VAT. But Duty is charged only on the intrinsic value, and is not put on Postage and Packing.
(I say GB because Northern Ireland is different than the rest of the UK in this post Brexit setup).