Here are some of my preferred A-level maths revision websites.
http://m4ths.com/teach-yourself-a-level-maths.html
This site is run by retired maths teacher Steve Blades and contains links to a great deal of instructional videos that offer a lot of detailed help on all areas of the syllabus.
https://www.savemyexams.co.uk/a-level/
Save My Exams offers help on maths and the three sciences in not just A-level but pre-U, GCSE and IGCSE exams and is syllabus specific.
https://www.physicsandmathstutor.com/maths-revision/
This site offers syllabus specific support and has solution banks and past papers.
https://www.biochemtuition.com/
Despite its name, this site has extensive support as well as copies of text books (copyright alert!)
https://www.examsolutions.net/
This website is highly rated by my students. It has instructional videos on examination past papers for mathematics A-level. It covers Edexcel, AQA, OCR and MEI syllabuses.