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My favourite A-level textbook is "Core Maths for A-Level" by Bostock and Chandler. It's aimed at students with only an Intermediate GCSE in maths and recaps a lot of algebra, so it's a good option if you might be a bit rusty. It covers the whole core curriculum, from basic algebra and geometry right the way through to calculus. There are lots of graded exercises, with a full answer key at the back in most editions.
The explanations are a bit terse, so you'll probably want to back it up with video lectures of some sort. There are tons of these available from Khan Academy and others. PatrickJMT on YouTube is very good, but do shop around and find someone whose style you enjoy. A search for any mathematical topic on YouTube will yield dozens of good results.
If it's been a while since you studied mathematics, you might want to pick up a GCSE revision guide and make sure you're confident with that material. One of the biggest mistakes people make in maths is to try and learn too much material too quickly. You can generally busk a topic even if your understanding of the prerequisite material is a bit vague, but eventually you'll come unstuck. Relatively minor misunderstandings and weaknesses compound quite quickly and a lot of people just lose all confidence in their ability to do mathematics. The importance of a solid foundation of arithmetic and algebra cannot be overstated. Don't move on from a topic until you feel that you've mastered it totally and could teach it to someone else. If at any point you feel uncertain about a technique, don't ignore it - go back and revise it until you've got it down pat.
An excellent option for building your general mathematical reasoning are the correspondence learning books by I.M. Gelfand, entitled "Functions of Graphs", "Methods of Coordinates", "Algebra", "Trigonometry" and "Geometry". They were originally written in the 1960s, for talented secondary-age pupils living in remote parts of the USSR who might have no access to proper mathematics instruction. The books contain a series of logic puzzles that require no prior understanding of mathematics, building an understanding of each topic from first principles. They are hard work, but extremely rewarding.