I'm not aware of any UK state schools that teach Japanese as a GCSE or A-level. That's not to say they don't exist, but I imagine the demand for Japanese teachers is very low, hence there not being a PGCE available for them.
Would you be able to teach French, German or Spanish (or possibly Mandarin?)- did you take any of them at A-level? If so, I would find a PGCE for one of these subjects and explain your interest in teaching Japanese also. I doubt the uni will have a placement school that offers Japanese, but you may be able to run it as a club etc.
Once qualified, it would probably be a case of finding a school you could persuade to start a GCSE course in Japanese. You'd need to teach your other language at KS3, I imagine.
If you *know* a school that teaches Japanese, then get in touch with them- it may be possible to do a schools direct course through them.
However, you need to be aware that being a Japanese teacher at secondary level in the UK is not a common job- and it may be worth exploring other options.