I'm probably not the best person to ask about this sort of thing, but I know fellow postgrads that live nearby to UCL that pay in the region of £550 PCM for a room in a houseshare. Of course, the further away you live, the cheaper it will/may be, but this is an approximate region for costing.
From what I understand, they found some sort of website where people were offering rooms in shared houses, and you sort of look around, then they pick which of the people they want to be their housemate, but there's a lot of trials for limited places.