Hi - I used to work for a Japanese company for 2 years in the Piccadilly area. Piccadilly/Mayfair is THE place for Japanese food. But as this was about 10 yaers ago, can't remember their names, which isn't helpful !!
As you are booking into a hotel, why don't you get a hotel in the Piccadlly or rather posh Mayfair area and walk around ? I doubt you will find a bad Japanese restaurant there, as most will be very up-market and have to be as they cater to the Japanese who work and live in that area (they are rather demanding when it comes to perfect food) !!
I remember one of the places my bosses took me to a couple of times was on Jermyn Street. If you really want to live it up you can get a private room with your own private chef who cooks your food right on you table. Jermyn Str. is just South of Piccadilly (parallel to it). It is where you find all the men's posh frocks (sorry, I mean shirts !!) and ties.