Are you doing A-levels now?
You can either apply directly to these international universities to do your full course there or do a study abroad year at a UK university. If you applying directly, I think you would have to get an equivalency certificate or some sort if the country does not recognise A-levels. Are you looking at the University of Seoul or Seoul National University?
I think the easier route may be to do a year abroad through a UK institution. I did my second year abroad, which fortunately for me was fully accredited as it was my university's abroad campus. We also had the option to do a year abroad in a tonne of other universities (including Seoul I think), but it was not accredited, so you would have to take an extra year when you came back to the UK. As far as I'm aware most universities in the UK offer study abroad options.
Hope you find your answers, studying abroad is really fun!