I guess it's kind of like how we have D and E grades at a-level/Pass grade at BTEC. Even though almost no uni or job would consider these 'good' grades or an actual 'pass' even, but we still have them to show that they're not TOTALLY useless at that subject, and do have some knowledge.
So I would say a 3rd is the equivalent of getting a D/E at a-level. You still kind of understand some aspects of the subject, but not as well as you could. An ord degree (so below 40% in 3rd year) is kind of like a U I guess, or I suppose you could also argue it's an E since you do still get an award. Depends how you look at it.