as it stands you will need to take a PCR or Lat flow test AND fill in a form saying if you have been to other countries in the last 10 days ( passenger locator form ) before you get on the place in Portugal ( it was in the last 72 hours, but they think that is due to change to 24 hours ) , then before the end of day 2 of your arrival in the UK you need a PCR test

from what i understand from my kids doing 2 tests per week, the free tests they ( the NHS ) send out are Lat flow tests, so that will not cost you anything