There is nothing definite about Brexit yet, but the government of Portugal has publicly stated that U.K. citizens who are resident in Portugal will not be changed by Brexit.
How can I keep living and working in Portugal as a British citizen after Brexit?
Is there any way for me to keep working and living in Portugal legally after Brexit? Has there been any agreement between the two countries to secure the status of people who already established a life here?
Due to the good relations that exist between the two countries, the Portuguese government has already made several statements to clarify that for British citizens living in Portugal, their residence will not be affected after Brexit.