You are Autonomo already, so that is the easy bit. The next thing you should do is find a good Asesor who is experienced in these things. Ask for recommendations, as you can bet your bottom dollar that if you ask the Asesor himself he will say that he is, only for you to come unstuck further down the line.
The next things is the question of whether you pay VAT in the UK for the goods that you have bought there. IF you are on the European Traders List you should not pay any VAT and so you should ensure that you are enrolled on this list as soon as you decide to import from the UK.
Then it is just a matter of paying your Spanish IVA on the sale of the goods - 18%
Also, IF you are charged VAT in the UK you can not then reclaim it in Spain.
This is a very confusing issue and takes some getting your head around, which is why it is so important to be sure that your Asesor at least has a handle on the situation !