Could i be a model in japan or south korea?

I think no. :D But, you can try....

There are plenty of foreign models working in Japan, there aren't as many opportunities in Korea. To get a visa to work as a foreign model you first need to be signed to an agency in the UK and build up a portfolio, then your UK agency will submit your portfolio to agencies in Japan and Korea to see if any of them are interested in representing you and sponsoring your work visa. Just compare the photos of the models during Tokyo Fashion Week https://www.google.com/search?q=tokyo+fashion+week&hl=en&client=firefox-a&hs=8ej&tbo=d&rls=org.mozilla:en-US:official&source=lnms&tbm=isch&sa=X&ei=iu0bUZbVBcmR0QWTsYGIDQ&ved=0CAoQ_AUoAQ&biw=1280&bih=535 vs Seoul Fashion Week https://www.google.com/search?q=seoul+fashion+week&hl=en&client=firefox-a&hs=Rfj&tbo=d&rls=org.mozilla:en-US:official&source=lnms&tbm=isch&sa=X&ei=ne0bUaOqAaag0QW9woHoBg&ved=0CAoQ_AUoAQ&biw=1280&bih=535 You see a LOT more Causasian models in Tokyo while in Seoul almost all of the models you see are Korean

You'd have to fit the beauty standards of that country. Honestly, they probably wouldn't accept you. As Asians, we want to see our own people shown as beautiful. We're tired of having whites set as the standard of beauty.

Why don't you just model in the UK? Modelling in Japan of Korea won't increase your chances of being a great model or famous.

nah, its not THAT easy to become a model in japan or s.korea, but you could consider china. They really like white people and since you're british its even better, (they respect us alot). But don't consider cities like beijing there are way to many people already there, maybe guangzhou or somewhere near there. And btw I'm not joking or trying to be rude about them liking white people its true, alot of my chinese friends have confirmed it.