You might check with the VeganSociety. There is a website listed veg roommates, etc. in England, but for the life of me, I can't find it. I came across it when we were preparing to move, and thought I'd bookmark it away for well, a post like this.
That said, you could try a post on Gumtree.com I've seen several posts on there, where folks note that they are vegetarian, etc.