Education in the United Kingdom: Is it harder to get top grades at top universities than at lesser universities? Why?

There shouldn't be a difference at all between say a Russell Group and a Post 1992 Uni. Any difference at all would be down to a university's grading requirements (e.g. When I was at uni - 2008-11 - some departments there would only award 5% a first, some had a mean average requirement of 57.5%, others a 90% pass rate with classification awarded - within reason of course!). This was depending totally on the nature of the subject, and what the uni deemed to be acceptable. In terms of cross marking, universities tend to make sure that standards are in line with those of similar research & ranking standards (e.g., my department sent our final year exams & dissertations for screening to Edinburgh & Southampton). It's just to make sure standards aren't way out of line.

It's getting progressively easier to obtain top grades at any UK university, see https://en.wikipedia.org/wiki/Grade_inflation#Higher_education. As to whether it's easier to get top grades at some institutions than others, it appears to be the case. If you pull the latest table of stat's, you can divide the percentage obtaining a 1st or 2:1 by the entry points for the institution / course. You'll note some institutions have a significantly larger numbers, from the calculation, than others. A small difference may be explained by the teaching methods, but there's quite some variance.

In the UK, if you didn't do your degree at a top university you are deemed by employers and universities alike as not particularly bright. Certainty not in the top 5%  The top universities are far better funded by government than the ex- polytechnics. In turn these institutions attract the brightest students.  Big name employers screen out graduates who didn't do their degree at a Top 25 university through the A level grading system; not knowing for sure whether a 1st from an ex polytechnic measures up. They do that because they know it's highly unlikely the graduate who got a 1st from a post 1992 university did as well in their A levels as the Oxbridge graduate with a 2:2.

It is much easier to get top grades at top universities. It is not that the examinations are easier or that the lecturers cheat, but a top university is only top if it provides an excellent education. The quality of the education is largely measured by the grades handed out. So if you get into Oxford and don't get a II.I you need to think of a good excuse. A really good excuse. Mind to be fair there are some seriously good excuses out there. Like the young lady who went up to read drama and had to have an exeat for most of her first term because she was starring in a West End play.

In short, yes. Aware of the disparity in aptitude amongst students, exams at lesser universities are comparatively easier. While the syllabus may be identical; the examination itself will be vastly different. However, I can only speak from experience as a Law student at a Russell Group institution and not for any other disciplines. I have friends and friends of friends who study law at (not to be undermining) significantly lesser institutions, and the method of examination is profoundly easier. For example, one student I know was given a multiple choice section for their exam, and another had a module split into 7 topics, all 7 of which were to come up in an exam in which only 3 needed to be answered, so only 3 topics needed to be revised. The level of detail required also is not the same. This reflects in the profession with many of the bigger law firms commonly using solely the Russell a Group and a few outliers as their pool of recruitment.