London is probably one of the easiest places for vegetarians.  Vegetarian London, published by Veggie Guides, boasts over 300 entries, so there's plenty around.

Cheap Eats

Leon Restaurants

Lovely chain serving cheap organic food with a wide veggie (albeit not vegan) selection.  4 locations in central London.

Veg Veg

A chain of Thai Vegan buffet restaurants that offer an all you can eat buffet for £5-£10 depending on time and location.  They also do take away boxes for £3 or £4.   They are located all over London, so look at their webstie to see where the nearest one to you is.

Diwana

A Vegetarian Indian Restaurant that offers an all you can eat lunch buffet for £6.50.  In the evenings it is menu only and an average main course is about £5.  They don't sell alcohol, but you can bring your own (only when eating from the menu, not when having the buffet) and there is an off licence next door.

Drummond St

There are a few reasonable priced Vegetarian Indian restaurants on this street, so definitely take a trip down there, if you like Indian food.

Red Veg

The veggie alternative to MacDonalds.  It a fast food restaurant in Soho, that is approved by the Vegetarian Society. 

 

Evening Meals 

Mildreds 

A vegetarian restaurant located in Picadilly, with many Vegan and wheat-free options, which are clearly marked on the menu.  They don't take bookings, so you may have to wait a while for a table, but you can wait in the bar.  Main courses are around £8.

The Gate  

Superb vegetarian restaurant (with vegan options) in Hammersmith. Courtyard area is great in the summer. Close to the riverside pubs near the Hammersmith Bridge.

Carnevale 

Cosy Mediterranean resaturant and deli near to Old Street and the Barbican. Pretty small, so booking ahead recommended.