You need an ETA for United Kingdom if you have a South Korean passport

From 8 January 2025, if you are visiting the UK for under six months, you will require an Electronic Travel Authorisation (ETA) to travel. Travelers don’t require an ETA if visiting the UK before 8 January 2025 for a maximum of six months. An ETA is not a visa. Beginning 27 November 2024, travelers can submit an ETA application online. Once the application is approved, travelers will receive their ETA confirmation electronically. Travelers visiting the UK for more than six months, for example to live, work or study in the UK, should check the source for more details. For travelers who are existing visa holders who live in the UK and use a physical immigration document to prove their rights, the UK is moving from physical immigration documents to eVisas. Travelers with existing visas should take action now to create a UK Visas and Immigration account to access their eVisa.