Apologies for being late to this very interesting thread, and thanks to @PerpetualOptimist for raising important questions around the best way to teach FM and its drop out rates. The competing claims are delicate, but as a former STEM admissions tutor I am always for having a good fallback. I have heard many, many stories from our applicants about how FM can go wrong in Y13.
The lower rate of FM participation amongst girls is a barrier to STEM education in the most elite programmes. When girls are genuinely more talented and interested in other things this is fine, but when they lose out on high paying careers that might suit them well because of external or internal biases it is not.
I was motivated to write because STEP and TMUA are mentioned upthread. They are becoming more widely used, especially for Maths admissions, as the A star grade becomes less of a discriminator. Several institutions offer a choice between them and the TMUA is significantly easier!
As PP said, STEP is run by Cambridge Maths who set the grade boundaries nationwide. They do this to control their intake, accepting only about 50% of offer holders.
This is of course their perfect right; my concern is that it is not widely known. Candidates from schools with a Cambridge tradition get substantial tuition whilst many others are like lambs to the slaughter.
TBF Cambridge warns them thar although STEP covers the material from their curriculum the problems are more challenging and the Cambridge STEP website provides good (free) resources. So do others. The free materials at the Advanced Mathematics Support Programme, are very good, and they run revisions at a low fee.
If your DC has Cambridge Maths ambitions, they should optimally start looking at STEP over the Christmas holidays in Y12 (when they may understand statements of a couple of the problems, at least). All DC but particularly girls need to know that success is a combination of talent and effort. The level at Cambridge Maths is very high, but there are few geniuses like the lad above.