Class of degree vs. job prospects

I wouldn't say so... getting AAA when you're at university, so your knowledge is possibly broadened from your course, as well as knowing 2nd time round the potential pitfalls, isn't the same as getting AAA first time round aged 17/18. Secondly 'special consideration' is available through the exam boards for illness, so it doesn't always count as a valid reason in the real world.
Good idea, but...
a) Where is the cut-off point? How do you measure which are the "top 20" unis? Places like Sheffield, Leicester, Newcastle, Loughborough, Southampton, Lancaster are in top 20 in some league tables, not in others, so if they're excluded even though they're top 5 for [certain criteria] it wouldn't work.
b) As I said it's not 100% exclusive to the top 20. They simply want the very best candidates, so the most outstanding people from Liverpool, Hull etc may have a good stab at it.
c) It'll look very snobby and arrogant - what about the AAA person who had to stay at home and go to their local uni for whatever reason...

People do need to become more realistic though, the 'truth' is only found out through rumours/peers/hearsay. The likes of Goldman Sachs get people with CDD, a Media degree and no financial experience applying on a daily basis.