Maths and computer science supercurriculars

Do wider reading around your subject area, work on some self-started personal coding projects, look at e.g. coding challenges/competitions and hackathons or robotics competitions with coding parts for example, maths competitions/olympiads, etc, are all possibilities. Note these are just illustrative examples, doing these will not guarantee you admission anywhere and equally, you can get into "top" universities for those courses without doing any of the latter (although wider reading is kind of a basic expectation for all universities).
That said I would make a couple points about maths and/or CS:
a) Maths at degree level is very different to maths at A-level (including FM). I would recommend exploring those differences a bit through your wider reading to understand what a maths degree is (and what it isn't!) and how that ties into your interests, strenghts, and preferences with regards to maths. If you really enjoy doing problem solving with (increasingly sophisticated methods of) calculus then you might find a course in e.g. physics or engineering fits your interests moreso than a maths degree for example! Also keep in mind you'll learn some basic programming skills (either formally or informally) during most single honours maths degrees no matter what.
b) While I've given several coding related suggestions above, it's important to bear in mind a CS degree is not a degree in programming/coding and often this can be just a relatively small part of the course (particularly at universities like Oxbridge/Edinburgh/etc, which often emphasise the mathematical and theoretical underpinnings of computers and computing). It's a degree in computer science after all - and this goes double for joint honours courses with maths which tend to focus on these areas even more. Bear in mind also a degree in CS is necessarily mathematical, so you wouldn't be "dropping" maths if you did a single honours CS course!
Also doing 4 A-levels will confer no advantage in admissions so unless your schools teaching of maths/FM is such that you would ordinarily take all of A-level Maths plus the exams in year 12 then all of FM and the exams in year 13, you can safely drop two subjects and it would probbaly be advisable to do so. Obviously you should aim to drop your 5th subject ASAP - I would honestly suggest dropping one now if you can.

I’m aware there’s no advantage to 4/5 a levels, but unfortunately if u want to take FM at my college, you have to do 5 in yr 12 and 4 in yr 13

5 in Y12?!? That’s simply a daft policy from your school. I’ve seen 4 been mandated (also daft and based on misinformation) but never 5 before.
Are you able to get your parents to convince the school to let you drop down to 3/4 A-levels whilst keeping FM?

Unfortunately there’s at least 50 other people in the same position (as they do FM) and I’m not exactly underperforming so I’m not sure how I’d approach that - my view is that if they can manage so can I…😅