I've seen the following scenario amongst more times than I care to remember.
X wants to do sciences, or parents pressurise X to do sciences because they are "employable".
X isn't very good at, or doesn't enjoy, maths and doesn't want to do A Level maths. They do a mixture of A Levels and/or BTECs that aren't mathematical.
X selects science that doesn't require A Level maths, which hugely limits choices (both of subject and course).
X gets place using non-maths A Levels and/or BTECs. X last studied maths at GCSE, and didn't do very well (as otherwise they'd have done A Level).
X arrives at university, finds that (a) the course is actually quite mathematical (statistics, often) because sciences almost by definition are (b) most of their peers have a maths A Level and (c) the university may have a "catch up maths" course but it's very hard for someone who hasn't student maths in two (or sometimes three) years.
X fails first year.
The reality is that in 2015, the concept of a "non-mathematical science" is almost an oxymoron. And a B or C at GCSE, two or three years ago, isn't a preparation.