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
http://www.westminster.gov.uk/leisureandculture/sports/porchesterspa.cfm
only about $50 a day for all day entry, to die for relaxation and beauty treatments there too.
To glam up for cocktails - nightclub style - Cafe de Paris and Elysium very cool, Friday night is a winner