Alexandra Helm

Hello, I was hoping to seek some advice regarding being self employed in two countries. I am a self employed dentist working in the UK 2 days per week, registered with HMRC. I am looking to work one day per week, again as a self employed dentist, in Spain. I currently travel every week between Spain and the UK. So I spend 3 nights a week in the UK, and 4 nights a week in Spain. In terms of foreign income in Spain, does this need to be declared and taxed in Spain, or via HMRC in the UK? Thank you.