The Schengen area, covering most EU countries, will be introducing the Entry/Exit System on 10th November, and the ETIAS travel authorisation in the Spring of 2025. These will affect all British nationals (other than double nationals and those with EU residency) to undergo additional formalities when entering the Schengen area.
The UK has announced that it will introduce an Electronic Travel Authorisation for EU citizens from 2nd April. See the article in The Independent of 11th September. This will be similar to the Schengen area's ETIAS and will cost £10. At the moment, the ETA only applies to certain Middle Eastern countries. The extension to most other countries, including the EU, has now been announced for April 2025.
Although this will not affect British citizens resident in the EU if they travel into the UK on their British passport, it will affect those EU-resident Britons who are double nationals but currently only use their EU passport and British citizens who have spouses or children who do not have British nationality or a British passport.
To find out more about how the ETA application will apply, go to the GOV.UK website.