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In order to qualify for non habitual residency in Portugal you would need to work in a profession that is classed as “high value-added activities”. Here is a link to the latest list of professions that fall within that category, https://www.plmj.com/xms/files/03_Novidades_legislativas/2019/010_outubro/NI_NHR-New_high_valueadded_activities.pdf.
In order to break UK tax residency you would have to be non-resident in line with the Statutory Residency Test (SRT). There are three tests where if you meet any of them you would become automatically non-resident, the ones applicable to you would be whether you spend less than 16 days in the UK in a tax year, or whether you work full time overseas.
Noting for the second one that this would have to apply for a whole UK tax year. But if it didn't apply for the whole first tax year you move but it did for the second you could potentially split the first tax year into a period of UK residency and UK non-residency, based on the date you move.
If you don't meet any of the automatic non-residency tests then you would look at if you meet any of the automatic resident tests, the most prevalent being if you spend more than 183 days in the UK in a tax year.
If you don't meet any of those then you would look at the sufficient ties test which determines your residency position based on a combination of how many ties you have to the UK and how many days you spend here.
The SRT is quite complex but I hope that gives an overview. If you need any further information please let me know.
The high value added activities exemption does apply to foreign sourced income, and provides a tax exemption from foreign sourced income.
In terms of UK tax on your employment income, you would only be taxable on your UK workdays if you were working overseas.
However a point worth noting is that Portugal may class your Portuguese workdays, working for a UK company, as Portuguese income.