"I'd go further and suggest doing A levels and trying to do at least the lower level of A level maths, because realistically the maths in the ATPLs is at the top end of GCSE skill level."
I have to disagree with this unless GCSE Maths standards have lessened significantly from the days when I did it…..and I only hold a GCE O Level (yes, that long ago!) at Grade C.
I teach ATPL theory in heavily mathematics dependent subjects and I would say that the required skills are in basic concepts such as:
Add, Subtract, Multiply, Divide
Simple Fractions
Transposition of Formulae
Simple Algebra
Geometry (Parallelograms, Circles and Triangles)
Area & Volume
Trigonometry
You certainly do not need to be able to carry out calculus, simultaneous or quadratic equations or spherical trigonometry.
The greatest failing I find is a lack of ability to carry out relatively simple mental arithmetic to approximate answers and an over-reliance on electronic calculators to find the precise answer to 12 decimal places, when it is unnecessary. The distance along a meridian between N 35° 05' 40" and N 52° 11' 55" is roughly 17° change of latitude or 1,020 NM. If there is only one answer in that ball park figure it must be correct - the fact that the actual answer is 1,026.25 NM is neither here nor there if the answers are well spread.
I would suggest a few months specific tutoring from a maths tutor who knows aviation rather than spending 1-2 years on an A Level. Not me by the way….far too busy personally but PM me and I will point you in the right direction.