Prior application for ETA is required for short-term visits to the UK for business or tourism, etc., when no visa is required. Visitors from over 40 countries and regions, including Japan, will also be required to obtain an ETA from 8 January 2025. Applications will be accepted from 27 November 2024. In addition, from 2 April 2025, ETA will also be mandatory for visitors from Europe, including the EU. (This does not apply to passport holders and visa holders from the UK, Ireland, and British Overseas Territories.) For details, please visit the website.
https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta#apply-for-an-eta