It is worth buying a copy of the BBC Vegeterian magazine. And check out the Vegetarina SOciety web site. There may be good ideas for where to stay.
Aunties are a good organisation to contact.
There are reasonabel and cheap restaurants behind Euston station. And if you shopin the West End, try Wagamamamas behind Slefridges or the Buddhist restaurant around the Oxford circus area - others worth checking out are near Liberty. There is a book on Vegetarain food in London. Restaurants tend to be quite scatterd though.