Arigato or 'thanky you' in Japanese is completely my favorite Japanese supermarket in London.
They have a great sushi / food bar where you can get fair priced sushi to eat in or a range of bento boxes, chicken karaage or gyoza.
The best thing about this place is the sweets fridge where they have a massive selection of Japanese treats including Pocky. If you don't know Pocky its bread sticks covered in different chocolate - sometimes you get strawberry pocky or sometimes dark chocolate or sometimes you get 'Men's' pocky which is dark chocolate with banana underneath - anyway this place always has the freshest flavors of Pocky.