On 11 January 2019, the Portuguese government-issued a contingency plan to reassure UK nationals of their rights after the Brexit.
If you are already a permanent resident in Portugal, nothing will change. The members will have the same rights and obligations before the Brexit. The requirements and fees to renew the permanent resident permit, as stated in Law no.37/06, will continue to be applicable.
In the case of the signing of the withdrawal agreement by the UK and the EU, the application of a resident permit won't change until the end of the transition period, which is 31 December 2020.
However, in the case of no-deal between the two entities, UK nationals who will be eligible to apply for a resident certificate until the end of the transition period must enter the country before the withdrawal date, as stated by Law no. 27A/2019.
According to the 2018 CIP Brexit report, "With Brexit, the UK will have more difficulties in importing and exporting to and from other countries in the European Union." Adding that "Portugal can replace other countries in the relationship with the United Kingdom market in the post-Brexit, assuming itself as a privileged partner of that region."