Hardest part of A level maths

The ones they have no interest in. If you have an interest, you'll find it easy, even if you have to engineer that interest!
Oh and year 2 integration

I didn't find differentiation hard at all, integration simply because we learnt it so quickly, at the same time as differentiation, and it is just a bit harder...

Other than that, not much tbh. Perhaps year 2 trig, because it's in integration a lot haha. Vectors are definitely easy.

I find with calculus, as I said, interest really helps. It'll allow you to properly understand it in a mathematical context, rather than an exam context. So when that really difficult question comes up, you have the edge.

I'm doing computer science, so somewhat similar =) I'd say the most important, at least for me, is linear algebra and probably calculus - for the maths modules.

To properly appreciate it, I'd say get interested out of school time. Before we'd first learnt calculus I'd heard about it so much I was really excited to finally find out what it actually was and how it worked. I've, since then, watched a few YouTube videos on it, explaining it in some nice alternative manners, without exams in mind, and there are some quite nice explanations.

I definitely recommend 3Blue1Brown's series on calculus, some really nice animations and explanations - without always referencing derivatives to be gradients of graphs. I find it slow at times, since it's split into manyyy videos, but if you pick the ones you want and have time they're super nice to watch.

Just things like that, doing maths problems outside of school, you'll come across a lot of knowledge that you otherwise wouldn't.