When looking for a restaurant, a good place to check is the TopTable website. You can book reservations, look at reviews and maybe snag yourself a special offer. Going for ethnic food is your best bet in London where food can be very expensive, ethnic food tends to be more in the afordable range.
Melati
At Melati restaurant you will find an excellent range of traditional Malaysian and Indonesian cuisine. Established in 1979, Melati is probably the oldest and most authentic Malaysian restaurant in Soho. Melati is situated on Peter Street, just off Wardour Street, and is very close to Piccadilly Circus, Leicester Square and Carnaby Street. It is roumoured that the Malaysian Ambasador to the UK dines at Melati. ($$)
For other Malaysian and Indonesia restaurants please check this list.
Pho Cafe
Pho opened in June 2005 as London's first Vietnamese street food restaurant with multiple locations across London. Of course you could also go to China Town and enjoy Pho at some of the Chineese and Vietnamese restaurants, but Pho cafe offers pho at locations outside of China town and they don't serve any Chinese dishes or offer a mind boggling menu of choices. The also don't buy in pre-packaged processed meatballs and frozen spring rolls. Pho cafe also provides food that is suitable for vegans.($)
Bali Bali
Bali Bali offers truly authentic Indonesian restaurant in the heart of London's West End. They accommodates many tastes, from the firey hot to the cooly mild; vegetarians, piscaterians and carnivores (all their meat is all Halal). For those of you who require a quick bite, Bali Bali have special set menus throughout the day. ($$)
Maoz
Maoz is a fast service restaurant serving authentic falafel and freshly made vegetarian food. The London franchise is located on Old Compton Street. Maoz serves the best falafels bar none, and their garlic sauce is to die for, also don't miss out on all their other toppings.($)