That was my thought too, though if there's not much on there...! Otherwise, Gumtree is a pretty good website and I guess you can post an ad looking for veggie housemates, making sure veggie is in the title.
There's no facility on Happy Cow as far as I know. The majority of the site is very USA-centric anyway.
Good luck with your search. You could maybe put up a card in some of the London veggie restaurants too.
Might be worth posting a message on the London group on Volentia, we've had people in a similar situation. It's a great networking site for veg*ns anyway, and has over 400 London members and over 13,000 worldwide...