The truth is there's hardly a six-former in the UK that doesn't have a crisis of confidence during the AS Level phase, in fact there is an incredible amount that leave before taking A2's. From my experience, it's the method in which it's taught- ie: the onus is much more on the individual to find out about the subject, rather than it being taught directly. Self motivation, planning & discipline is the key.
Just about everyone doing a degree has a second year crisis, when it sinks in that attending lectures & partying isn't going to get you a degree- consolidated research & reading is the route to knowledge & the ability to express yourself on paper.
The obvious answer is to gain your qualifications whilst young- it gets harder as you get older, not least because you take on financial commitments which mean you have to earn to survive- academics become secondary.
In contrast to that, instead of £30 per week EMA, in the Corps you could earn £25,000 whilst you could have been gaining A Levels. You could earn a further £50, 000 in the Corps, together with 5 years promotion seniority & hands on practical experience, possibly a second specialist trade - whilst you could have earned a degree which teaches you the theory of the subject & £15,000 student debt. At today's rates that's about half the cost of a house you could have earned if you joined at 16 as opposed to studying for a degree.
When all is said & done it's your choice, there's no right & there's no wrong. Good luck.