In London? Your own home. That is unless you want to get a private tutor, but otherwise, group classes will get you nowhere.
Some useful resources for home/self study..
Grammar -
Tae Kim's Japanese guide to Japanese grammar
Kanji - Heisig's Book Remembering the kanji, flashcards,
reviewing the kanji,
iKanji Touch (for iTouch or iPhone)
Dictionary:
Denshi Jisho - Online Japanese dictionary
Vocab - Flashcards, maybe
smart.fm - The place you go to learn.
Other possibly useful resources (in no particular order).. Lang8.. AJATT (
All Japanese All The Time Dot Com: How to learn Japanese. On your own, having fun and to fluency. )... Google Japan
Google
Etc...