As someone who has been self-teaching A Level maths for the sake of learning, I really hate the Edexcel specification for calculus. It skips over so many proofs and concepts that a deep understanding is all but impossible unless you go out of your way to learn from other sources. In addition, the order that material is presented makes things needlessly more difficult.
As an example, you are introduced to the number e early on in C3 by being shown the derivatives of 2^x, 3^x, etc. and being told that because of a noticeable pattern, there must be such a number as e. The problem with this approach is that at this stage, you don't even know how to even derive 2^x or 3^x. Moreover, deriving them requires you to use logs in base e, making the entire proof circular.
Another example is the fundamental theorem of calculus. The Edexcel textbooks simply skip over it, giving only an extremely brief explanation on how to find definite integrals. So you're left wondering just why taking the definite integral of a function will magically find the area under a curve.
To compound matters, the Americans teach calculus in a completely different order, and teach concepts we don't need to know for A Levels, such as discontinuous functions and taking limits from both sides. So when you try turning to internet resources focused on the American curriculum, you'll occasionally find you're expected to have knowledge of things you either haven't learnt yet, or won't learn for A Levels.
All in all, calculus as taught by Edexcel is extremely frustrating if you like understanding things.