Hey,
I have just finished my A Levels which included A Level maths. Here are some of the things I found the most useful:
1: PhysicsandMathsTutor - loads of past paper questions, PDF cheat sheets and past papers. I can tell you now that past papers will be your friend for the next to years!
2: ExamSolutions - Really well-made, detailed tutorials. Index of videos on the website, including past paper Qs with detailed walkthroughs. I'd say this is THE best free resource that's out there for A Level Maths, hands down.
3: Uplearn or Tailored Tutors - I bought an Uplearn subscription (for maths and chemistry) only last March, and it is definitely helpful. Although I find they can break it down a bit too much and really string it out. Good if you use it from the start as you get a really solid understanding. But since I only used it for 4 months during the last stages of revision it wasn't quite what I needed. I also bought a subscription for Tailored Tutors Biology, and I think it is far better than Uplearn and feels a lot more human, as there is a really good support network. I should think the Maths course is just as good as the Biology course. May be worth doing a free trial for it? The only thing is with these two is the cost - it is pretty expensive. I think for 4 months Maths and Chem I spent about £350, and TT is £80 a month. So it is pretty expensive, especially for a long period of time. But if cost isn't an issue I would probably recommend it, but check it out first.
In terms of textbooks, I just used the pearson Edexcel ones, which my college lent to us. They're pretty good, BUT, and take note of this... Do not just rely on the exercises when revising maths. They are good to do in class, but once you have done exercises and understood the topic, move straight onto exam questions - in the first test I did in Yr12 I spent half term doing loads of textbook exercises, felt really happy and prepared and the way I revised was correct... got to the test and came out with a U. So don't make that mistake!!
Also, and I know this is the most clichéd thing anyone can say about A Level Maths... YOU MUST ASK QUESTIONS IN CLASS! This is something I didn't do pretty much at all until halfway through Yr13. Its so important to understand everything, and asking questions makes connections in your brain and helps you understand other topics as well, so don't be put off by any arrogant classmates, which you will likely come across doing A Level Maths.
Anyway, hope some of that was helpful. Good luck with A Level Maths... It is hard, but its also a really rewarding subject. So stick at it when it gets tough and get help as soon as you need it!!
Let me know if you need to know anything else