Nah, it's basically discretionary, in that A-levels and GCSE's (inc. GCSE English and Maths) are a requirement, but what you take for your A-Levels is up to you. Certain choices may be preferable to an employer, but that's it.
Beyond that, you could consider taking a few hours towards your PPL; a few hours is enough to show enthusiasm, but not enough to gain bad habits.
Check EasyJet's careers website; that'll give you info on their recruitment process, which seems one of the better options. NetJets are another example; for that program, you pay in installments.
I do not know what to tell you about the specifics of a job as such, but I think one like that would be incredible. I am not ballsy enough; I'd have to be a flight attendant or something. This might give you some insight:
You can pick up a lot of bad habits getting your PPL. But possibly looking into joining the RAF cadets might be worth your time. You get free lessons toward your PPL, and there's alot of adventure/leadership training on offer, I'd reccomend it.
Subject wise, ideally you want physics/mathematics, maybe IT. They all go hand-in-hand anyway.
I don't know much. My friend had graduated with an engineering degree but his dream was to become a pilot. He was trying to get on a training course with BA plus taking flying lessons etc for many years but he would always miss out on a vacancy because military pilots were preferred.