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:

hotcourses or the London-specific Floodlight.  Cactus Language Training offers a wide range of language classes internationally.  Gambatte, ne!

The Institute of International Education in London (IIEL) www.iiel.org.uk

Offers a range of Japanese classes at all levels, taught by native speakers.  They also offer ‘Tailor-made Japanese Tuitions’ if you need to brush up quickly.  IIEL also trains Japanese teachers, and offers free Japanese lessons taught by trainee Japanese teachers at their centre in Greenwich.  

Alpha Language School www.alpha.ac.jp

Likewise, Alpha Language School has two programmes:  Japanese lessons and Japanese teacher training lessons.  Located in central London.

UIC www.uiclondon.com

UIC has Japanese classes from beginner to advanced with several weekend courses also available.

The School of Oriental and African Studies, University of London (SOAS) www.soas.ac.uk

SOAS offers many different Japanese courses from beginner to higher advanced, and including full-time Japanese Teacher Training courses and the certificate and Diploma in Modern Japanese.  

If you are studying for the JLPT (Japanese Language Proficiency Test) which is held yearly in December at SOAS, you might like to take their preparatory classes.

Japan Foundation’s London Language Centre www.jpf.org.uk/language

The London Language Centre has an extensive library available, and offers talks and courses to those interested in learning Japanese.  There are also Japanese Refresher courses available for teachers of Japanese in the UK.