Depends on what courses the Uni counts as contributing to the classification. Usually this is 3rd and 4th year courses. Here, it's as simple as this:
70% + 1st class
60-69% 2:1
50-59% 2:2
40-49% 3rd class
In the meetings however if a student is on the borderline (say, got 69.1%) then the lecturers discuss the student and if they feel it's right, they make the case for that student to be bumped up.
This is why, in large part I say to all students on here - go to your lectures. Be engaged and talk to the lecturers. If you do that, and you make a good impression, they'll go to bat for you.
If you do it all from home and don't go in, as many people on here have said they do, you won't get that.
We had a student with 49.8% and nobody made the case for him to get a 2:2. He's getting a 3rd. That's because he didn't engage with the lecturers or the courses.