On the consequences of UK’s withdrawal from the European Union, you can find detailed information provided by the European Commission on travelling between the EU and the United Kingdom from 1 January 2021 here: Notice on travelling between the EU und the United Kingdom after the end of the transition period.
Do I need a visa to enter the United Kingdom?
Travelling to the United Kingdom from 1 January 2021: EU citizens can continue to travel to the United Kingdom without a visa for a stay of up to 6 months. From 1 October 2021, a passport will be required for EU citizens to enter the United Kingdom. Those EU citizens who fall within the scope of the Withdrawal Agreement (persons who have "settled status" or "pre-settled status" in the United Kingdom under the "EU Settlement Scheme" and cross-border commuters) may continue to enter the country with either a passport or an ID card. This will apply until at least 31 December 2025. As of 1 January 2026, the United Kingdom will be free to no longer accept national identity cards for the purpose of entry if these cards do not contain a biometric chip.