Hey, it may vary according to which University you are at (I know that Bradford give distinction,merit or pass instead of the traditional system of 1st, 2.1, 2.2, 3) but the general trend seems to be some kind of average of your performance over the three years - many places don't include first year marks you just simply have to pass.
So at our place, it was 10% 2nd year, 30% 3rd year and 60% 4th year. I got year averages of high 2.1 in 2nd year, and 1sts in 3rd and 4th year and came out overall with a 1st but did not get a 1st in every single exam I sat - my lowest mark was a catastrophe of a physical pharmacy / aseptics in 2nd year but it didn't affect my overall outcome too much.