You can be awarded a pass degree (a degree without honours) if you get below 40% by special dispensation.
I know for PPE, E&M and I think quite a few others it's done that for a certain grade on a paper, it's 40, 50, 60, 70 (3rd, 2:2, 2:1, 1st respectibely) as grade boundaries. However for you're overall classification you need to get at least two papers in that particularly grade or higher, and have an average mark that is:
So if you get an average of 68, but have two papers above 70, then you're awarded a first.
There is another rule though that if you get 3 papers marked above 74, you automatically get a first by "minority excellence", provided the other 5 papers are a 2:2 or better. However nobody's yet to get 3 papers over 74 and not have an average above 67 yet. The rule is there so that people who are *really* amazing at management, for example, can get a first, even if they're not particularly good at economics.