From the British Embassy in Zagreb:
Have you travelled to the UK using your Croatian/EU ID card?
Entry requirements have changed!
From 1 October 2021, EU, EEA and Swiss nationals can only travel to the UK using a valid passport.
ID cards will no longer be accepted as a valid travel document and it will not be possible to enter the UK using one.
This will not apply to anyone who has applied to the EU Settlement Scheme.
Those who are eligible to continue to use their ID card for travel, should ensure it is registered to their UKVI account to avoid any unnecessary delays at the border.
Those who do not have status under the EU Settlement Scheme (EUSS), can enter the UK for holidays or short trips for up to six months as visitors, visa free.
However, now freedom of movement has ended, those coming to the UK to work or study must prove they meet UK entry requirements.
EU, EEA and Swiss nationals can only cross the UK border using a valid passport ( not an ID card ) which should be valid for the duration of the visit.