Some observations, based on personal experience and interacting/working with people in some of these areas:
1) The entrance procedure for getting into a top university is very noisy. It is hard to predict how good someone is just from their A-level grades and an interview (even private sector job interviews are notorious for being unreliable). Therefore there is no reason to believe the average student at Oxbridge is much better than the average student at any other top Russell Group. What you do find though is that the majority of _very_ good students do end up at Oxbridge (Im not talking about those that just get top A level grades since that isnt too difficult, I meant the superhuman ones that have IMO Gold medals and suchlike). So the top 5% of students at Oxbridge will be a lot better than the top 5% of students at (eg) Imperial, but that doesnt mean the average student is much/any better.
2) Because of the above, its very silly to say that a 2:1 from Oxbridge is 'better' (in terms of ability) than a first from a top Russell Group. The 2:1 Oxbridge student is average by Oxbridge standards, while the guy with a first might have been good enough for Oxbridge but didnt get in due to the noisy entrance procedure, or another reason. Without any more information, who knows
3) When it comes to private sector recruitment at most financial service 'elite' jobs (eg investment banking, management consultancy, etc), having a first rather than a 2:1 doesnt matter that much. The things that matter are a) institution quality, b) experience (internships, etc), c) extracurriculars, d) personal characteristics. Whether you have a first or a 2:1 comes quite far down the list. The point of getting a first is if you go to a lower tier university and need to show that you are better than your institution quality, it has less importance if you are already somewhere elite.
4) Having a first rather than a 2:1 does matter for jobs that recruit mainly based on raw intelligence rather than extracurriculars and personal skills. Investment banking is not one of these jobs. Things like PhD programs, some tech startups etc are (although even for these the difference between a first and a 2:1 will be less important than other things like having a strong portfolio of research experience, extracurriculars, etc)
5) For investment banking, whether you are at Oxbridge/Imperial/LSE makes very little difference, all of these are viewed as pretty much equal. When it comes to city jobs, you will find the majority of people come from those 4 places (although there are also lots of people from other universities)
6) There are some super-elite jobs which recruit only from Oxbridge (sometimes only from specific Oxbridge colleges). These are often private equity firms, which don't have formal graduate programs and only recruit 1-2 people every year, so there is no point in them casting the net wide.
7) There are also some other jobs where having an Oxbridge background helps a lot, because the jobs are mainly about schmoozing with clients who have also went to Oxbridge (and private school), so having the right personal background matters a lot. These jobs are typically things like big law and private equity.
8) As someone said above, jobs which involve 'selling' their people to clients also favour institutional prestige. Being able to tell your client that you will get a team of Cambridge graduates working on their project sounds better than having a team of Nottingham grads. However, the difference in prestige in this context between Oxbridge and the next tier down (Imperial/LSE/UCL/etc) is probably less than people here think.
9) There is a lot of comradery between Oxbridge alumni - graduates from those two universities tend to have a lot more pride in their institution than graduates from places like Imperial/LSE (mainly of whom actively resent their university). This means that you are possibly more likely to be favoured during networking/interviews if the other person is also an Oxbridge graduate.
10) There are no jobs which have a £60k starting salary straight from undergraduate. However a £60k+ all-in compensation package is realistic at top tier investment banks/private equity/etc (this includes salary + sign on lump sum + bonus)
In summary; having a 2:1 from Oxbridge isnt 'better' than having a 1st elsewhere nor does it show more intelligence, and noone really views it like that. However, there are some situations where the Oxbridge brand name may help you, particularly if your job mainly involves interacting with clients who themselves are largely Oxbridge alumni. Typically, the more technical (=scientific, mathematical, etc) your job is, the less likely anyone is to care about your university background. Oxbridge bias is more common in jobs like media, law, etc, whereas noone would care in engineering, compsci, etc. Also, the jobs which do have a strong Oxbridge bias are often looking for people from a certain upbringing (private school, nice accent, able to act comfortably in formal settings around those from elite backgrounds, etc) where the Oxbridge degree is just the cherry on top. If you arent from that background, then you probably arent going to land a private equity job out of undergrad just because you want to Cambridge.