The Bar is too complex a profession to be pinned down to specific criteria for selection.
For instance, the Criminal Bar may well be more open to someone with a 2:2 from a good (1994 group) University who has been an active debater with proven experience (e.g. President of their Society at Uni) and good post BVC work posts.
Granted, the Commercial crowd don’t tend to do anything but continue to ensure that everyone on the white wood board outside Chambers has MA (Oxon) or (Cantab) after their names. But then if you wanted to do Commercial you’d already know that.
The other overlooked aspect is luck. This is a tricky one because there’s no manual, blog or self-help guide to increase your proportion of it. Persistence seems to increase luck’s frequency of appearance though!
I should know, I am a 2:2 holder from a 1994 Group uni who is due to start a (criminal, obviously) pupillage in October 2010. I passed the BVC in 2007.