Thanks to Gigi for putting this in the comments.
Whereas, on 31 December 2020, the transition period for the United Kingdom’s withdrawal from the European Union (Brexit) ended, as of 1 January 2021, the appointment of a tax representative by individual and collective taxpayers, with tax domicile in the United Kingdom, became mandatory since it was considered a “third country”.
The deadline for the designation of a tax representative by citizens and legal entities that are registered in the Portuguese Tax Authority’s database and have their address in the United Kingdom, has been extended to 30 June 2022 .
See here the order 150/2021 XXII.
In case of doubts, you can contact the Tax Authority through its service e-Balcony .
You can also contact the AT Call Center: +351 217 206 707 or the Taxpayer Registration Services Directorate dsrc@at.gov.pt
For information in English, click here .