It goes back to my point about looking at each candidate individually, rather than having an arbitrary cut-off. Postgraduate courses get significantly fewer applications than undergraduate, and it is reasonable to expect that given people are much more selective and deliberate in choosing PG courses, that the admissions tutors put the same effort back into selecting a class.
That having been said, I don't study law (although I did consider it), but I am a postgraduate student. There is no link in my experience between the undergraduate institution people have come from and their current performance. JG thinks that some departments "prepare" students better for the BCL than others. But it's still down to individual ability, which I think MUST be considered more highly than university of origin, ceteris paribus (academic achievement).
Based on the experiences of Laura and Julia, the BCL seems like it's hard for the sake of being hard, and that it only takes 1st class graduates in order to maintain/justify its prestige. I have a degree in IR and I'm an adherent to constructivist theory--behavior is shaped by how one constructs their own identity. The Oxford BCL has constructed itself as "the greatest postgraduate law degree in the world" or whatever, and it must maintain this identity--otherwise people wouldn't slog through hell for a year and sacrifice their health just to get the degree. They'd go to London, or hell, Harvard, and still get a fantastic education, without the expectation that you can't have any holiday time or whatever nonsense I've seen about the course.