I did A-Level maths myself using textbooks (edexcel). I was able to complete it in one year doing 3 exams in Jan and 3 in June. Just find a school that allows for private candidates and pay the exam fees which will be about £100. I did C1-C4, S1 and M1 by the way.
I can sign up with AQA for June. Those private schools wouldn't happen to have Spring exams?
edit: nvm