Regarding fluency, a lot of it comes down to how well you use your year abroad. If you spend it making friends with locals, and make Japanese your daily language, you'll come back to England in your final year with a good level of fluency. If you spend your year abroad hanging with other foreign students, you may well come back worse at Japanese than when you left. Furthermore, most people I have spoke to say that none of the university courses really guaranty /full/ fluency. Rather, a good degree in Japanese will take you to the stage where if you were then to spend an extra year in Japan you would be able to become very fluent. I think most courses take you somewhere beyond Level 2 of the Japanese Language Proficiency Test, while Level 1 is an understanding of the language close to native.
Also, I wouldn't pay too much attention to the tables, even the subject specific ones. Go on the websites and look at the courses in detail. Cambridge is the most focused course, as I understand it, in the first year you do language and 'East Asian history', where as after thay you can do about two topics a year alongside language study. SOAS is broader, with each year having 4 'units', so you could potentially study a wider range of topics than at Cambridge. Leeds looks very broad, having a /points/ system, with some courses only taking 10 points out of... 80? 120? something like that, a year.
So yeah, do some research and see where appeals the most, 'rankings' be damned.