1. ExamSolutions- There probably isn't anywhere else you could find this much content dedicated to A-Level maths. He also has content on really specific topics. I also recommend checking out his website, he has tons of other resources on there. 2. TLMaths - Again, some really good A-Level specific content. 3. Starfish Maths - Covers more basic concepts, but a great place to start if you're completely new to the topic.
4. Eddie Woo - Another great place to start learning new topics. I don't recommend him for A-Level specific stuff, but he has tons of really interesting topics covered.
5. The Organic Chemistry Tutor - His math videos are just as good as his chemistry ones. He has a ton of them too, you'd probably find a video on any math topic on his channel.
6. Brian McLogan - There's some really good examples of specific types of questions here, not A-Level specific but still extremely helpful.
when you're stuck on a question
Worked solutions to your math problems: 1. Symbolab Math Solver - Worked solutions to whatever problem you can't seem to solve. This one is my favourite because it can handle more complex problems too. Other alternatives include, 2. Photomath - It's only a mobile application at the moment, but works great otherwise. 3. Mathway - Not my favourite, but still worth a mention. 4. Microsoft Math Solver - Doesn't offer full worked solutions, however it's still great for checking your answers or finding videos and more practice questions similar to what you've entered.
Graph plotter 5. Desmos - Really simple and easy to use
Best Math YouTubers (in my opinion) 1. Flammable Maths (strictly for entertainment purposes) 2. 3Blue1Brown (is more complex, but I've found some of his more basic videos understandable) 3. blackpenredpen (really helpful for A-Level too)
YouTube Playlist of 'must-watch' math videos: Click here.