Re: Do I need Japanese to practice in Japan?
There are countless foreigners in Japan who speak no Japanese and in some cases, very little English, and yet they manage to do aikido with only a few minor problems. You've probably already encountered the overwhelming kindness of this country. There'll be no shortage of people to help you out. Around Tokyo, there should also be no shortage of English speaking locals as well as other foreigners who can speak some Japanese.
One thing I've found is that doing aikido will help your Japanese anyway and in turn your Japanese will help your aikido. You get more chances to talk to native speakers, you get immersed in the language and culture and you make friends with people who expose you to more Japanese in other situations. So rather than worrying about Japanese as a barrier, preventing you from learning aikido, I'd urge you to look at aikido in Japan as a way to improve your Japanese and aikido at the same time.
Carl (a fellow Brit)
PS: PM me if you want to go to the Hombu sometime -- I'll be down on business a couple of times in June.