I would seriously think about doing a-levels if you can because you have the option of doing a degree/higher apprenticeship if you wish or at least by that time, you will be 18 so you have far more opportunities on the railways.
In regards to driving, mainline is 21 but tube driver is 18. Depot driver is usually 21 although under certain conditions, I think it can be 18 although rare. (From what I remember reading, I think it has to be restricted to depot and it must not be managed by NR although I could be wrong). Possibly heritage railway might be 18(?) and then you have a much better prospects of becoming a mainline driver at 21.
I am not sure when you say you want to be a HST driver, that you are referring to Class 43 or fast trains in general; I too would love to be a Class 43 driver but even in about a year's time when I come of age,
if (I'll put this in bold before the "you have no chance of a driver's job at 21" brigade come along) I can become a driver at 21, most of the HSTs will be gone or at least from mainline services. Until then, I will stick to my simulators
