The UK has officially left the European Union, and the transition period will end at the end of 2020.
The UK parliament formally passed legislation to confirm the UK's departure from the EU, and the withdrawal agreement earlier this year.
Although officially outside the EU, the UK will continue to remain a member in effect until the end of 2020 - a so-called "transition period". During this time, EU citizens can continue to move to the UK to live and work, and UK citizens can continue to move to EU countries too. Both the UK and EU have agreed to continue to treat each other's citizens in their own country on the same basis as at present after the end of 2020. This applies to any citizen moving and establishing residency prior to the end of 2020, not just to those citizens who are already living in the country. So there is a still a window for UK citizens wishing to move to Portugal under EU rules to do so.
The UK has strongly indicated that it will not extend the transition period, even despite suffering heavily with Covid-19 and with businesses and government being heavily occupied with the impact of the virus and the required changes to operations. So it seems likely that this may be the last chance many UK citizens have to exit the UK.
I have been dealing with a number of couples where one partner is a UK citizen and the other is a non-EU national. Under EU family reunification rules, the non-EU partner is allowed to move freely to Portugal with their UK spouse. But with the end of the transition period fast approaching, there are worries what will happen if the UK citizen registers their residency before the end of the year, but the non-EU partner can only get a SEF appointment in 2021. The good news is that the transition agreement states that citizens of the UK who move to Portugal (and other EU countries) and establish residency there before the end of the year will be able to bring in immediate family members (such as a spouse, or children) indefinitely on the same terms as at present, providing that relationship existing prior to the end of 2020 (the UK has agreed reciprocal arrangements for EU citizens in the UK).
In other words, if you are married, or get married before the end of 2020, and the UK partner establishes residency in Portugal before the end of 2020, the non-EU partner will be able under the rules to join their UK spouse in Portugal under similar arrangements to now, effectively as an EU family member.
This also applies to unmarried couples with a long term relationship (over 2 years), however in those cases it is harder to prove. With marriage, there is documentary proof so it is typically makes the immigration process less stressful and dependent on the immigration authorities' whims.
However, from the start of 2021, UK citizens resident in Portugal who then meet a non-EU citizen and marry, will not necessarily be able to bring them to Portugal on the same terms as they can now.