Why study A level maths? You'll earn more!
A 2013 study showed that students who take A level maths will earn on average 13% more at age 30 than students who don't. There's also evidence to show that taking other STEM subjects could increase it still further.
There are more details in this Institute for Fiscal Studies paper from 2013, complied by Martin O'Connell. Also some fearsome equations to show just how much more you might earn if you also study science or computing at A level. (But be warned: you might need to already have a Maths A level to be able to interpret them...)