Hi andreventura,
If you plan to live in Portugal on a long term basis, you are likely to be deemed tax resident in Portugal and therefore liable to pay Portuguese tax on your UK employment income.
At which point HMRC will authorise your UK employer to apply an NT code.
You should advise HMRC of your change in residence circumstances by completing form P85.
Or the Residence pages of your Self Assessment (SA) tax return, if your are within SA.
Employees working abroad
Get your Income Tax right if you're leaving the UK (P85)
Thank you.