As of today, Friday 1 October 2021, most Europeans will need a valid passport to enter the United Kingdom (UK); national identiy (ID) cards are no longer accepted as a valid travel document.
This post-Brexit requirement applies to most citizens of countries in the European Union (EU) and the European Economic Area (EEA), as well as Swiss nationals.
The British Embassy in Luxembourg confirmed that most EU, EEA and Swiss citizens must present a valid passport when travelling to the UK or risk being refused entry.
Exceptions apply for Irish citizens, EU nationals living in the UK under the EU Settlement Scheme (or having applied to the scheme but not yet received a decision), British citizens living in Gibraltar and those with a cross-border work permit, among other groups.
Further details are available on the UK Government website: https://www.gov.uk/guidance/visiting-the-uk-as-an-eu-eea-or-swiss-citizen?fbclid=IwAR2BuxsGrQqbMMF3rKS51nbYhhCx2teGKiFD6y7OCIbw-kr0fBFr4CC4Skw.