For people living in or visiting central and outer London, we have been visiting these (exclusively vegetarian/non-chain) restaurants for around 30 years and have always found them good:
Vijay's Chawallah, Green Street, Forest Gate
Diwana, Drummond Street, Near Euston
(both more like upmarket cafes than restaurants but excellent food and very good value)
and for the last 5 years:
Tibits in Heddon Street, off Regent Street
(extensive buffet, food charged by the weighed plate) Very nice atmosphere and good value for the West End. I've just rolled back and seen this has already been recommended.
(I think Carluccios, though not exclusively vegetarian and a chain, is pretty good)
I find many places that serve vegetarian food but which aren't exclusively vegetarian are generally disappointing - and I agree about the often tasteless mush otherwise known as mushroom rissotto.