There are many places to study Japanese in London.  You'll find Adult Education classes locally will sometimes offer Japanese, but often only at a beginner's level.  For higher level courses, you might have travel further afield. To begin your search, try:

Do you want to find like-minded people who appreciate both Japanese and British culture and speak the two languages?  Try a conversation evening.  This is London – so you’ll meet all sorts of different types of people - from Anime artists to Zen masters!  Great if you’re new in town… or just want to meet some new people.

The London Japanese Conversation Meetup Group