I have been working in Spain for the past 5 years but I would like to register as self-employed again in the UK without moving back to the UK. Is this possible? Or legal?

You live Spain and ought to declare your income and pay tax in Spain. It is not possible i.e. legal to register as self employed in UK, unless you move back to UK.

If you are self employed and live in Spain you should register as such here not in the UK. I expect there are particular jobs where you might be self employed based in the UK and then working in Spain for some project but in general I would have thought the answer is No...

Thank you for your replies,
I forgot to add that I am leaving Spain to live in central America. It is very difficult to register in Nicaragua as a resident, I know other brits that live there and they told me that they are registered as self employed in the UK working for on line as a freelance esl teacher. But there I will be earning very little but if I register in Spain I will have to pay a fee every month for being self-employed and I will be paying more than I earn.

I would suggest you contact HMRC. I have always found them very helpful.