Maths at uniWatch
If you look towards the top, Warwick has 82% of students getting a 2:1 or above, Oxford 75%, and so on, despite probably having harder courses.
The answer to your second bit - courses are not uniform in difficulty (think entry requirements) and some will be more demanding than others. (though if you look at those with similar entry requirements, they're generally not going to be that much different) There is external moderation of courses but there are no standard specifications like at A-level, it varies. This is hard to gauge just by looking at numbers, because a course's difficulty roughly speaking scales to the ability of its cohort, which will vary. So they shouldn't be wildly different.
This shouldn't be something you base your choice on, there wouldn't really be any difference between a 2:1 at any of these unis. Go with the course/uni that looks best and don't worry too much about it.