Although I cannot speak for every low-ranking university, I go to a non-Russel group middle of the table uni.
Most of my friends from school go to better universities so I know people from Russel Groups and Oxbridge and such.
It comes as no surprise, that the calibre of student at my university is lower than those at better universities, myself included however I try and make the most of the hand I've been dealt, but a lot of people in my class don't do this, most people try and do the bare minimum to pass. So when we eventually graduate, and the 'average' student struggles to find a good graduate job, is that really because of the university's poor teaching and guidance or is it because the students?
I know people at other unis, who have had a bunch of internships at big companies, volunteer, do charity work. I feel they'd probably still be a success at my university, whereas if you got someone from a low uni and put them in Cambridge, I doubt they'd do much better.
Obviously, some unis may have better ties to certain industries or companies, or might be targeted by certain employers. But I feel for the most case, you could probably achieve relative success at London Met if you were bothered to.