It depends where in London you will be, but you could try any of these:
Kikuchi (near Tottenham Court Road station)
http://www.london-eating.co.uk/4925.htm
Asakusa on Eversholt Street near Camden.
http://www.london-eating.co.uk/2679.htm
Tomoe in Marylebone. (Read the blogger's review at the link below)
http://www.london-eating.co.uk/35283.htm
Soho Japan on Wells Street, off Goodge Street.
http://www.london-eating.co.uk/3076.htm
There are hundreds of great recommendations on this thread on Chowhound:
http://chowhound.chow.com/topics/390388
Happy sushi munching
