Thanks for your replies.
Tomu, my main reason for going to Japan is to experience Japanese culture, starting with the Japanese language. Aikido is just an extra thing I'd like to do while I'm there. It's an important extra thing, because I do love Aikido, but it's not my reason for going.
Matt, I'll probably want to continue with my Aikikai style training, and won't want to put the hours in to do the Riot Police course. Do you know people who've wandered from dojo to dojo, or have been to a "sports college"?
I lived abroad for a while elsewhere (Chicago, actually) and after a while I knew I definitely didn't want to stay, but it was still very worthwhile going. I expect to feel the same about Japan. I suppose you have to be something of a masochist in this way to do martial arts training in the first place.