S1 was my lowest too - 85 (not quite dipped into the 70s, but it was still pretty disappointing).
Yes, some maths applicants have 600 UMS. And? I honestly don't think that matters too much. Yes, you're not going to have the "wow" factor, but you'll definitely get an interview, and if you impress at interview then they'll make you an offer.
Remember that you drop to 95% by only answering three questions wrong. Personally, I think it's silly for people to get worked up about it - that's something as careless as putting a minus sign in the wrong place (I actually did this with my C4 exam. Funny thing is, I did the question right the first time and then went back over it and thought "Shouldn't that be minus?"). But just because I think it's silly to get worked up over it, doesn't mean the cambridge admissions tutors agree with me...