There are lots of inexpensive ethnic places in Soho - off Shaftsbury Avenue near Piccadilly.. some good Japanese etc. For an inexpensive lunch, the Patisserie Valerie chain is OK and the sandwiches from Pret A Porter (better than the ones in NY) are interesting and not costly for London. For unusual Indian food, I like The Cinnamon Club in Westminster - the surroundings are very British and nice for tourists. It's mid-priced although there are often deals there from www.london-eating.co.uk. Your friends should read through the discussions at www.chowhound.com. A lot of great restaurants are mentioned and talked about. A lunch at the Borough Market on a Friday or Saturday is fun - lots to choose from and good coffee at the Monmouth Coffee House across the street.