It depends on if you want to save time or not. There are many things in GCSE maths that I do not think you would need, e.g. drawing a box-plot.
I would be tempted to just start at AS Level and fill in any gaps you need to before moving onto A2. Calculus can be extremely easy and extremely hard, so you would need to determine what level of calculus you need. There is a big difference between integrating a trigonometric function and deriving Kolmogorov equations.