I don't mean to piss on your bonfire but...
IMO you should put all your time and energy into your A-levels then worry about a job afterwards. Working in the aviation industry isn't all that it's cracked up to be with awful hours and equally as terrible pay.
Your best bets are to finish college, get decent grades and wait for the summer temp jobs to appear with various handling agents. You won't be earning more than £7 p/h in them roles and you'll be worked to death, trust me
I work with some people who have over 30 years experience in the aviation industry but are stuck dispatching for £8p/h because there simply aren't any jobs going anywhere for experienced staff, let alone people with no experience. Our station has just announced another 50 redundancies...
Stick to college and keep an eye out on the various websites and you may get lucky.
In regards to staff travel...
I can't speak for other handling agents but at Servisair one of the few benefits we have left is staff standby travel on TOM/TCX/MON. You can also try your luck with other airlines too, if you contact some directly they'll give you aviation staff discount.