Japanese Food Available in Supermarket in UK
Some people may think Japanese cooking is not so easy to start with because of all the ingredients you might not be able to get hold of outside of Japan. But, I'm so glad to say most of major supermarket in England nowadays have "oriental" section where you can find most of the necessary flavors that Japanese cooking require.
I'm most impressed by the effort made by Waitrose to introduce Japanese and Chinese food. They have products from ClearSpring which is a major Japanese Food (organic) distributor in UK for many other supermarkets. But Waitrose produces their own Japanese/Chinese related food and those products are much cheaper. I certainly recommend for you to visit Waitrose if you want to buy small quantities of Japanese food to start with. Here are some very useful products that I found very useful if you keep in your cupboard or fridge for a touch of Japanese flavour in your kitchen.
Japanese Supermarket around North London
I'd say it is so possible for us to make Japanese food easily with local supermarkets but if you like to be a little more adventurous, there are Japanese supermarket available in London. Those places will have wider range of Japanese food imported from Japan so will certainly be more authentic.
Hopefully my research will reach to wider area in the future but for now, here are some of the Japanese food stores available around London.