I have did a bit of tutoring for struggling students at a 1960s university currently ranked roughly mid-table. I attended a Scottish Russell Group University and one half of Oxbridge. I'm sure those of you will know which institutions I speak of. I had offers from others, Durham and St Andrews, and undertook research for a while at the other half of Oxbridge as well as another couple of RG universities, and will be teaching at an ex-poly next year as well as a Russell Group university as a graduate teaching fellow. I think I know the course structure and assessment very well at three universities, from those regarded on here as middling to very good to the world's best, and know the standards fairly well at another three or four.
My experience has been:
The difference in standards have been exagerrated on here. From Oxford/Cambridge to bottom, there will be a significant gap- I know that one university asks for a dissertation of half the length of another, has fewer hand ins, fewer exams and no assessed presentations. The gaps in the middle, from the Russell Group to Oxford/Cambridge, from top 20 to top 2, is very, very small. Most students will be amongst the best in their high school year, most will have at least AAB or equivalent at the end of school, most will be able students. The difference is in the outliers. There are some extraordinarily talented students at both institutions, but some of the real geniuses go to Oxbridge. Similarly, the tail of poorer students is shorter there. This isn't to say a tail of poorer students that I think would struggle elsewhere don't exist, or that this tier of exceptional students makes up more than 2-3% of the total. The tail was longer again at the 1960s university, but still the minority, and they still weren't bad, and I found the material they were teaching to be pretty good.
In terms of standards, they're rigorous from what I can see, but different. One puts all the emphasis, or at least virtually all of it (barring the dissertation), on a series of final exams, sometimes two a day for five days, and your degree is based on that alone. The other put 65% of the emphasis on exams, but 20% on coursework and 15% on the dissertation, and the exams were sometimes two a day but no more than six in a week. The student at one might find one system harder than the other- with continuous assessment over two years you can't afford a string of bad grades- if the 35% that isn't on exams ends up with 2:2 scores, you simply will not get a first, so you have to be on form for two years solid. Again, some put less again on exams- at the other university there was only a half on exams- but this meant you couldn't afford more than a couple of bad bits of work if you wanted a first.
This brings me to point. You cannot say a first at one university is less than another, because they're often different, and the student is still getting top marks based on the criteria set out by the university. Who is to say they wouldn't have got those marks elsewhere. The difficulty of entrace is a non-issue. In the St Andrews forum the other week I was told by someone that her tutor had given her the paper he himself had sat at St Andrews in the 1990s as a practice exam- but back then it was BCC to get in and now it's AAB. The difficulty is still the same, as are the expectations. You would rightly expect more students to do well as the standard of entrant goes up though. It doesn't mean the graduate of the 50th ranked university is weaker than the graduate of the 5th. At one point, I was thanking my lucky stars that I had a string of good coursework grades before I went into the exam- knowing I had a high first for 20% was a good buffer to begin with. At another point at the other university I was happy that my string of near-misses didn't count for any marks, as it would all come down to the exams, and luckily that also went well for me. It's too hard to draw any more specific conclusions than that.