Self employed income in two countries

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.

Hi,
You will need to review the guidance on statutory residence and take the tests to determine your tax residence position:
RDR3 Statutory Residence Test
You will also need to review article 20 on other income and possibly article 7, if you have premises, to determine who and how your self employment income will be taxed. The treaty is reciprocal, so you can interpose UK and Spain as the 'contracting state' and 'other state', to see the position from both sides.
2014 UK-Spain Double Taxation Convention
Thank you.