Saw a security guard at White City Westfield with a dog the other day, for no apparent reason. They are very much fake cops, and rely on (a) the area that's owned by Westfield and the area that's public not being displayed at all - nothing is signposted and there's a large area by the tube station which may or may not be public space, I have no idea - and (b) the fact that there are often real cops there too.