I'm from Portugal and have a valid citizen ID card. Will I need a passport to spend the last weekend of February in London as a tourist?
According to the UK government website, nothing much will change for you as a citizen until at least 2021.
That means you will not require a passport for traveling to the UK after Brexit if you already have a valid EU Identification Card.
What you’ll need to enter the UK If you’re an EU, EEA or Swiss citizen
What you’ll need to enter the UK will not change until at least 2021.
EU, EEA and Swiss citizens can continue to travel to the UK for holidays or short-term trips, without needing a visa.
You’ll need to show a valid passport or a national identity card if you’re a citizen of either:
an EU country Iceland, Liechtenstein, Norway or Switzerland There will be no change to travel document requirements this year. We may stop accepting national ID cards for entry to the UK for EEA and Swiss citizens after 2020. We’ll announce further details, including the date for this change, in advance to allow travellers good time to plan their trips.
If you begin living in the UK before 31 December 2020 and have status under the EU Settlement Scheme, you’ll be able to use your EEA or Swiss national identity card to enter the UK until at least 31 December 2025.
Travel within the Common Travel Area will be unaffected by these changes.
Does 31 January change anything?
Yes. Although everyday life will remain the same and the UK will remain in the single market and the customs union until the end of the year as part of transition arrangements, the withdrawal agreement will be a legally binding international treaty.