Which course should I apply for university?

You could ask to self-study D1?

Do they do FP1, 2 and 3 then? I'd do S1 and S2 and M1 and M2 then see which you prefer; will you get a choice?

Maybe, but I don't even know if I'll end up studying Computer Science; I'll have to do further reading over summer. D1's all about algorithms, if I'm correct?

In my school, normal Mathematicians must choose straight away between Mechcanics and Statistics, but Further Mathematicians don't have to choose until Year 13.

Oh, our A level maths students can do M1/S1 or D1/S1 or D1/M1 or M1/M2 or S1/S2 but we do have lots of groups ... (they could do D1/D2 but I wouldn't suggest it as stats or mechanics is usually a better choice)

Thank you for your help!

Finally, is an EPQ worth it (my school doesn't even offer one)?

I think it depends on the courses you are looking at - it's not worth changing school just to do one.

Which courses out of those four listed would you say it'd be worth doing (in the odd case my school lets me take one independently, if that's even possible)?

They seem to be aimed at humanities type subjects. In maths you'd be better off self-studying D1 - that's just as impressive.