I have worked in SPain, and there is no issues for UK registered Electricians working there.
However if this is paid employment/self employed then you do need to register at a local Police Station in order to get an equivelent of our NI number. You can work without this, but you will be a fool as without it you will not be able to get Insurance cover and should you have an accident you would have to pay for your medical care, even though your a UK citizen, as you would be deemed as working illegally.
You need to know that they do not have the wholesaler network like we do in the UK, suppliers are few and far between so you would need the financial clout to keep a reasonable stock of the most common items needed..such as sockets, switches, roses etc