The Japan Foundation has a
list.
Most courses seem to be a dual honours degree rather than being Japanese only.
In a fit of fantasy that I could become a mature student
Cambridge's Oriental Studies seemed to be the closest to a Japanese only course. And it doesn't hurt that it's one of the UK's big two universities.
Of course these days you have to think what your return is going to be after you invest your £10k to £15k tuition not to mention all the other costs.
I'm all in favour of learning for learning's sake but the harsh reality is you need to get a job that'll use your studies at the end of it. Or somehow be creative enough to combine your interests and ambitions with making a living.