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From January 8, 2025, Japanese nationals will be required to apply for and receive prior approval for an Electronic Travel Authorisation (ETA) when travelling to the UK. (*Excluding the traveler is transiting without entering the UK)
As for visas, short-term stays of less than six months (tourism, visiting relatives, etc. – standard visitor), business, short-term study, etc. will be able to enter the UK without a visa in principle.
For more information, please check the UK government website.