Lancaster ranks the highest out the the 3 on the league tables, but how accurate are the league tables?
What league tables? The ones appearing in UK newspapers (which are strongly influence by student satisfaction, a very unreliable measure of course quality), the ones for international rankings of universities, or the ones for research quality?
As an academic in this area, I would say that Lancaster Maths is undoubtedly weaker than Nottingham and Exeter in terms of research quality, depth of course etc. Lancaster have however put a lot of effort into moving themselves up in the UK newspaper league tables, by focussing on measures that are valued in these league tables.
If your DD lies Lancaster the most, and wants to be in that part of the country, fine. But she shouldn't choose it thinking that is the "best" of the three. I would rank them as Nottingham, Exeter, Lancaster, in terms of course quality.
For Maths all three are likely to take one dropped grade, but none of them will allow anything below A in maths - although since she is taking FM it is unlikely she would get below an A in maths anyhow.