Presently, union citizens can enter the UK with both their national identity cards or their passports and they do not need a visa to do so. Although 31 January 2020 is the day the UK shall officially leave the EU, it will be followed by a transition period, in which the rules of entry and exit shall remain unchanged.
According to the agreement between the UK and the EU, this transition period ends on 31 December 2020. The Joint Committee (comprising representatives of the EU and UK) may extend this transition period one time with an additional 1 or 2 years. As a result, the current system could hold out as late as 2022, but for now 31 December 2020 shall be deemed the relevant date.
Come 1 January 2021, – assuming no extension takes place – it will be entirely up to the British Parliament to determine the conditions of entry and exit into the country, specifically whether or not a passport and/ or visa is required.