Personally I think it depends, to get into competitive jobs such as law and investment banking, getting a 2:1 from a top 20 uni is gonna be better than a 1st from a very poor uni.
Just to give u an example, a friend of mine, he got a 1st in Economics from Westminster, he re-did his A-levels and got 3As and he has done quite a bit of work experience as well as learnt a foreign language (German). He however got rejected from all 20 of the investment banks he applied to, none even gave him an interview.
In fact he said that nearly all of his colleages are facing the same problem.
On the other hand I know ppl who got 2:1 from from Imperial, Kings and UCL in un-related subjects (Engineering, Computer Science and one in Physics) who have been accepted into top Investment Banks.
As I said before it depends, some employers rather look at ur degree classification, but in very competitive jobs, employers are able to be very choosy as there are so many graduates to choose from.