From 2021
For stays of no more than 90 days in 180, you will require an on-line ETIAS visa-waiver, currently around £7 and valid for 3 years. To obtain that you would have to indicate your fist port of call, declare serious convictions within the last 10 years (20 for terrorism), you may have to declare serious medical conditions, prove you have medical insurance and also at point of entry prove you have sufficient funds and have a return ticket.
There is still time for the UK and EU to come to an agreement to return to passport-only entry, but the UK seems to want to make entry as difficult as possible for EU citizens by requiring full passports (as opposed to ID cards) and our own electronic entry system.
Although, of course, and EU's not wanting to go through all that can enter Ireland (Rep) on just their ID card and wander into the UK unchallenged.
Priti Patel hasn't quite got her head round that one.