Not London, but there's a nice all-you-can-eat sushi restaurant in New York that I've been eating at for over 20 years. Been through three name changes and countless price increases , currently about US$40-45 including tax/tip. Of course it's not the most amazing sushi, but it hits the spot and, in 20 years, I've never gotten sick. Lol. It's all made to order off a menu, nigiri or lots of hand/cut roll options. A bit heavy on the rice of course to fill you up and if you leave too much over they charge you extra.