The British government is gradually introducing the requirement for an electronic travel authorisation (ETA). EU citizens will need an ETA to enter the UK from 2 April 2025 if they do not already have another residence permit/visa. A corresponding application can be submitted from 5 March 2025 and should be submitted with sufficient advance notice (at least three days). If you enter the UK before 2 April 2025, you do not need an ETA to leave the country. Once issued, the travel authorisation is valid for any number of entries for a maximum of 180 days within a period of two years. Information from the British government. https://www.gov.uk/guidance/visiting-the-uk-as-an-eu-eea-or-swiss-citizen