I wouldn't necessarily argue against courses at the 'elite' institutions are harder than elsewhere but I have a few points to make.
I achieved ABB at A level, a decent but unremarkable set of results and ended up at York uni for my first year of undergrad physics. There was a friend of mine on my course who had A*A*A in his A levels (his third A level was also much tougher than mine), but when the end of year results came out, we actually had the same mark pretty much - he averaged 72, I averaged 71. I moved to Liverpool for my second year (where there's no noticeable change in standard between them in 2nd year work), but the same has happened this year, but this year he got 81 and I got 79, albeit in slightly different modules. I also happen to know that he's worked harder than me. It just goes to show that A level performance doesn't show how good you'll be at uni. There are similar cases of low A level scorers getting top marks at uni, and vice versa, just from people I know.
Also, while revising for my astrophysics paper in May I stumbled across a paper used at Oxford in their equivalent module. The Oxford paper was actually a 3rd year paper but that's beyond the point, they just do the modules differently, but the questions in the Oxford paper could well have come up in my exam (and very similar ones have in the past) because the standard was the same (I did this paper as part of my revision and had no major problems). Again, I know this isn't a comparison between a bottom ranked uni and Oxbridge, Liverpool is still a Russel Group uni and such, but from some comments you get around, you'd think it was a different qualification.
Like I said, I wouldn't argue that there will be differences in standards between some unis, but A level performance doesn't always carry over to uni due to the change in teaching style (I imagine especially for subjects with fewer contact hours, where a lot of material is self taught), and at the very least, standards aren't as different as they're made out to be.