A colleague of mine told me about one of her best employees-- he worked on the trading floor at JP Morgan in London. Very working-class Londoner--- perhaps a Cockney-- who had never been to university. During one of the interviews, a snotty Oxbridge MD started quizzing him about his A-levels, especially in math(s), which he didn't have. At that point, young miracle man leaned over and said, "Look. If I need maths, I've got a computer on my desk. And if it breaks, I'll buy another one."
I suppose one way to deal with this situation would be to say when asked, "my undergrad wasn't up to where I wanted it , which is why I'm glad I got that distinction in X at the University of Y. Now I feel more ready to take on Z." If there's some special reason you didn't get a 2:1, you might mention that, but probably best to keep it positive and forward-looking.