Since elderly South Koreans were all once Japanese, I still have some relatives that can speak and write in Japanese as well as some that still live in Japan and teach in Japanese. Many of my friends are also working or studying in Japan, mostly in Tokyo.
How much can you afford? Living a little farther from the downtown Tokyo can reduce the monthly renting cost. In general, Tokyo is much cheaper than Seoul, per income. Also cheaper than Shanghai for the relative buying power. As long as you don't have 2-meter leg, space shouldn't be a problem.
Clothes and food are often imported from US, Australia, China, South Korea, southeast Asia, etc. Supermarkets are well developed, at least much better than South Korea though there are still a lot of "mom-and-pop" stores. Taxi, subway, restraurants are expensive compared to most other countries. Hamburgers and internet are cheap.
Should ask the Japanese embassy in the UK. It's relatively easy for South Koreans (compared to getting a visa to live in the US or in the UK.)
White Caucasians are looked UP in Asia if you know what I mean. All peoples and individuals are racists to some degree. Racism starts from ignorance and lack of touch with other peoples and other cultures so it depends largely on education, intelligence, integrity, travels, etc.
Nothing you said seems to be of ill intention.