I don't agree - a 3.5 GPA is not equivalent to a 1st, even if you take into account those variables (I've worked in university admissions). Getting 80-100% on exams in the US is not the same as getting 80-100% in the UK university system; it's not so much about one being "harder" than the other, it's simply because they have completely different grading systems. e.g. In the UK a dissertation with a mark of 85% would generally be considered an outstanding piece of work, maybe even top of your class, whereas in the US this same percentage is about a B (so a good piece of work, but by no means the top of the class). Or as another example, a 64% in the US is pretty bad, whereas this is a solid grade in the UK.
The original poster was correct, a 3.5 GPA is considered to be broadly in the 2.1 category. I agree that there's all sorts of variables that make these things difficult to compare so there's no exact conversion, but I've never seen 3.5 GPA considered comparable to a 1st.