Living in spain, self-employed in UK

I am from the UK, and back in Feb 2013 my partner was offered a job in Mallorca, Spain. He is now a resident in Spain. When he moved over I did too, and started working part-time for 6 months, earning just under €3000 in total.

Since then, I went back to the UK and started freelance writing and have registered in the UK as self-employed. However, now as I work from home, I want to move to Mallorca again.

I have an NIE number from when I worked in Mallorca before, but I never applied for residency. Do I need to apply for residency even though now I am registered as self-employed in the UK at my UK address? I want to keep UK residency as I don't plan on settling down in Mallorca.