Difficult question as many of the chain places such as YO SUSHI and ITSU are fairly low quality affairs but at a reasonable price. Other Japanese Restaurants especially the authentic places with qualified Japanese chefs can be a little pricy but definitely worth a trip in entertaining clients or for a special celebrations.
There one call Asakusa around the Camden area. There is a review here
http://www.timeout.com/london/restaurants/reviews/895.html
Been there several times and its always a great evening.