I don't know of any all you can eat sushi restaurants in London. Certainly not central London. If you want something cheap but ok, then there are a few take out style chains - Yo Sushi, Wasabi and Itsu. There is also a more premium restaurant group called Roka which is quite good and Shoryu (a bit cheaper than Roka). Both have central London locations. For a cheap(er) dinner I generally go to Tombo Cafe in South Kensington. For something special head to Akira in Japan House.