All nationals of the European Union will not need a visa for tourist trips to the UK of short duration (less than six months). People making short business trips will also be exempt from visa requirements in the UK (simple professional visits, training, conferences, seminars, etc.).
The European health insurance card will no longer be valid in the UK. You will therefore need to have private medical insurance to travel to the United Kingdom.
Until 1 October 2021, it will still be possible to enter UK territory with a valid identity card or passport. After this date, the identity card will no longer be sufficient to cross the Channel and you will need to have a passport.
Please use the check-list on this link to find out the rules that will apply to your specific situation: https://www.gov.uk/transition-check/questions