One possibility is the Intercontinental Park Lane. i stayed there during the early 1980's quite often and they had an excellent complete Japanese breakfast that came through room service. i remember it well. The fish soup was delectable. They've changed much since then and are no longer the same quality so I'd be very surprised if they have it any longer.

The Hyde Park Hotel (now MO- BTW, there's also a new Hyde Park Hotel, not related) was also one of my haunts then, but I remember no Japanese breakfasts. I think I'd remember.

I do not remember beer dispensers in London.

I'm betting it was the Intercontinental, but I am afraid it helps you not at all to choose today. Nobu and the Metropolitan are favorites of mine even though Nobu (the restaurant) is hardly Japanese (although Nobu, the man, is) but is actually Peruvian-Japanese fusion, excellent all the same and the Bento Boxes are a very good room service choice.