I'm sure most of you know this is going on, but just in case...
This year it's in Trafalgar Square.
Japanese Martial arts has its own stage. As well as Karate (Goju-ryu), there is Iaido, Koryu Bujutsu, Kendo, Judo and Aikido.
Anyway, I thought I'd share.
All the best.
http://japanmatsuri.com/get-involved