London has become a haven for vegetarians.  Even the Sainsbury supermarkets now serve up delicious vegetarian prepared foods, and all of their offerings are marked with a large green "V" if they are vegetarian.  Similarly, the Marks and Spencer supermarket chains have many inexpensive and great-tasting vegetarian prepared salads. The Pret a Manger and Eat fast food chains, which are on every other block now, also have many vegetarian soups, sandwiches, and other options.  Here are some great choices for veg restaurants:

The Gate, Hammersmith, 51 Queen Caroline Street London, W6 9QL (very gourmet, fabulous presentation)

The Gate, Islington, 370 St John Street, Clerkenwell, London EC1V 4NN (different menu but just as fabulous)

Gauthier Soho,  Soho, 21 Romilly Street, W1D 5AF - A rare opportunity to find vegetables given the same star treatment as meat in a luxurious fine-dining environment, featuring a dedicated vegetable tasting menu. Book well in advance.

InSpiral Lounge, Camden Town Market, 250 Camden High St, London NW1 8QS (serves a great authentic British breakfast with veggie sausage from the Swiss company VeGusto that tastes as close to the real thing as is humanly possible; lunch and dinner offerings are excellent with some raw choices if that is your thing; comfy location overlooking the canal)

Mildred's, 45 Lexington Street, London, W1F 9AN (wide ranging menu with daily specials; truly delicious food and comfortable place)

Food for Thought, 31 Neal Street, Covent Garden, London, WC2H 9PR (emphasis on quantity so go if you're hungry, but the seating is all in a rather uninspiring basement)

Sagar Covent Garden, 31 Catherine Street, London, WC2B 5JS (and two other locations) (good, if not great, Indian food) 

For dessert, absolutely do not miss the weekend five-course dessert menu at William Curley, 198 Ebury St, Belgravia, London SW1W 8UN, or the plum tarts at Pain Quotidien.

If you take a weekend trip to Bath, the best vegetarian food in England is at the Acorn Vegetarian Kitchen, 2 North Parade Passage (just off Abbey Green), Bath BA1 1NX

Three other restaurants that an observer in 2011 on this page said served great vegetarian food.  

Manna Vegetarian, 4 Erskine Road, Primrose Hill, London NW3 3AJ

Maoz Vegetarian, 43 Old Compton Street, London W1D 6HG (veggie Mediterranean fast food, e.g., felafel)

Blah Blah Blah, moved in 2012 to Richmond, a far west suburb of London, 157 Heath Road, Twickenham

Others that have been well reviewed:

Vanilla Black, 7-18 Tooks Court, London, EC4A 1LB

NOT recommended are:

Tibits, 12-14 Hendon Street, London, W1B 4DA, this is a buffet with nothing interesting or inspiring, and expensive for what you are getting.  

Diwana Bhel Poori House, 121 Drummond Street NW1 2HL 020 7387 5556, the food is greasy if cheap and the seating arrangements are uncomfortable

Amico Bio in Bloomsbury, 43 New Oxford St, London WC1A 1BH, this is just mediocre Italian without the meat

Neal's Yard Cooperative, 2-6 Neals Yard, Covent Garden, London, WC2H 9DP, although this is listed as "veg-friendly" on Happy Cow and as "inexpensive," it is actually neither.  Expensive tourist trap with very few vegetarian possibilities.

Blue junction, 1 Crown Lane, Morden, Surrey. This place is located opposite Morden tube station and has been a wonderful place for a superb variety of vegetarian and pesketarian dishes. A real must