The impression I get is that a person who gets a good grade at a top institution would easily be able to get the same grade (or better) at a lesser institution. Whereas a person who gets a good grade from a lesser institution would not necessarily be able to get the same grade at a top institution. I would assume that it is easier to get a Distinction at Manchester Met than at the University of Manchester
There are two main reasons that I think this:
I studied Maths - and I've seen Maths exam papers from my own university, and from other universities, where at each one, you need to get 70% of the raw marks available in order to get a First. And it's quite clear to me that some universities consistently set harder exams than others, where it is more difficult to get 70% of the marks. So it follows that it is harder to get a First at those universities. It tends to be the universities with the better reputation that set the harder exams.
Also, I get this impression judging by many people I know who have gone to different universities, the grades they have achieved and the relative academic ability of each one. For example, I know someone who got a 2:2 in Law from Cambridge, and another person who got a 2:1 in Law from Queen Mary's. If you had known both of these people at school, there is no way you would have ever expected the Cambridge one to get a lower result than the Queen Mary's one, if they were taking exams of equal difficulty. And there are several other instances like this.