Hi,
I used to have a company in spain and I would like to warn you it is very expensive to set up, close down and run (you still have to pay the 250 euros SS before earnings)
I am now in a similar situation to you and have decided to register as a sole trader in the UK. Although it is not fully correct to do this I want to do things legally without being robbed by the government. If you are charge before earnings this is nothing but robbery in my book. I also cannot afford to pay 250 euros for the right to be able to earn money.
Also as for claiming costs etc in Spain nobody really nows what they are talking about, including the tax office and the laws are contradictory. When ever I had a tax question I would ask 5 different people (no exaggeration) and I may get three different answers. So I had to choose the answer that got the most people behind it.
Spain is a nightmare to pay tax and do BOOKS in.