Personally I wouldn't make A/S level maths mandatory, instead I would change the curriculum of GCSE maths to bring in a lot of A/S material on applied mathematics - particularly statistics and probability. Personally I think these subjects are essential for understanding things which are happening in everyday life - whether it's about enabling people to read through the statistical analysis presented by, the media, or whether it's about understanding how to manage their finances, or being able to understand basic concepts like inflation, risk, economic growth, etc. It would also ensure more time at GCSE level is spent teaching critical thinking and analytical skills.
Not that I have a problem with pure mathematics, but when there's a choice between spending time teaching subjects like Calculus which most people aren't going to use in their later life, compared with teaching applied maths which is relevant to everybody, I'd choose the latter.