If you like Japanese food, then Wagamama for sure! There are several locations since it's a chain. I ate there so much this summer hehe. There's also a really good crepe place at Gabriel's Wharf (over by the Tate Modern) in South Bank.
And Sainsbury's grocery stores have really good grab and go sandwiches.
How I miss London!!! If you like Chinese there is a fantastic Chinese restaurant in China Town called China China and the food is soooooooo good!!!! For an elegant meal Le Gavroche is not to be missed, the food is absolutely delicious! Selfridges serves some great salted beef sandwiches too!
For cheap regular food for real down to earth people, lots of Londoners go to West End kitchen or Stockpot, prices are about $10 for a main course.
Best Italian, is Il Paglaccio a family run Italian place on the borders of Wandsworth and Fulham on 184 Wandsworth Bridge Road, I go there twice a week whenever I am in London. Price about $15 for a main course, but tastes like eating at the finest restuarant in Sicily!
Of course I recommend Wagamama's too! Yummy food but a tad too westernized if you are used to more authentic Japanese food.
Best Chinese though is Royal China in Queensway, their signature dish is lobster with fried yellow noodles, just amazing. Expect to see Qs of people down the street for Dim sum on a Sunday and also some movie stars there too. Flash your designer gear there after spending a day at Porchester spa