Sorry to to extrapolate a bit on the FM question.
There's some debate as to whether FM is harder than maths.
In theory it isn't - it's still an A Level. Rather it's simply an extension of maths (covering additional topics).
However, some (though not all) of those topics are more complex - thus more challenging.
Speaking to the school, the biggest issue with FM, especially if your child is also doing physics is that 75% of their workload is then super maths intensive.
If, like my DS, your child is not only good at maths but actively enjoys it, then that's a great thing.
On the other hand if they are good at maths but don't really have a passion for it, then to have so much of your workload for 2 years focused in that area can be a huge demoralising slog.