Oxford/Cambridge have a reputation and can pick the crop of students.

When it comes to Russell group or a polytechnic, no one really cares. The university ranking system isn't based on how good a candidate would fit in a workplace (they would argue they aren't a trade school).

A 1st or 3rd class degree only really factors at HR level when they filter candidates. I actually helped kill that policy in my workplace because a passionate 2:2 is far superior to an disinterested 1st class degree candidate. Once you have a couple years experience in your field no one even looks to see if you have a degree.

I suspect your find your friends earning decent money did a vocational degree and/or had a firm idea on what career they wanted.

Quick salary rises are generally a result of either a stressfull/long hour job (e.g. stock trader) or a a market with a massive shortfall of qualified candidates (e.g. software).

You need to figure out what your interested in/want to build a career on. We can't really tell you want that is.