For Law at Cambridge (in the first year) you can get a 1st with two papers being 1sts and two 2:1s, so technically you could get a 1st overall for the year with an average of 65% (i.e. 2 x 70%, 2 x 60%). Apparently last year the rules were changed so that both the 2:1s had to be above 65% to get a 1st overall, but they changed them back because it screwed so many people over - a guy in the year above got two 1sts, a high 2:1 and then one at 64%, so missed out on a 1st for the year....
So yeah, here at least it is possible to get a 1st with under 70%, although not on any particular paper, just overall.