To teach in the UK you need a degree or NVQ level 5, and you need to do a postgraduate teaching qualification if your degree is not in Education ie B.Ed. As you progress along this route you will be able to ascertain where your interest lies and so get the right course elements to equip you say for teaching 16 - 19 or whatever. Being practical, you will need to work out how you'll make your living - there aren't that many places that teach Japanese, so are you going to research where these are and make them targets for jobhuntng, or will you teach something else as well so you can look for work in a larger number of places.
Japanese and Arabic were very trendy in the 1980s in the UK - now the fashion is for Mandarin Chinese.
Some private language schools will accept you with less qualifications than degree and postgrad but the pay will be lower.
Lastly, have you considered this the other way round, namely teaching English to Japanese students? There is a well-organised programme for this:
http://www.jetprogramme.org/