OK, let me put this into perspective for you...There are so many people in your position! (Me included) You've made a big step by starting out on PPRuNe.
I recommend you keep an eye on all recruitment, at your stage, just for interest rather than applying. Get to know how things work through researching - I believe that the BA sponsorship - when it returns or whatever - is maximum age of around 28 or so rather than 21.
Get flying! If you are earning £150 a month, then that gives you nearly 2 hours a month...If you don't need to spend your cash on other stuff.
I'm 17, and I earn approximately the same - I've just started flying, and when you do, you'll realise what an important move towards the career it is.
Also - Keep your head down at school/college - It's by far the most important bit. Get some good AS Level grades in the bag, and then you'll have less chance of dropping grades when the harder work comes along.
Remember - British Airways isn't the only sponsorship! There are plenty of other great opportunities. Consider the part-sponsorships, if you are guaranteed a job at the end, it's well worth loaning the initial cash!
Being shorter on cash than a lot of people around you (in flying anyway!) can be a bit daunting, but if you have the resolve to get there, you will... If you want a chat - e-mail me.
Regards
Mike
(I'll try and update my website ASAP so those in this position can get some info without having to start threads that go unanswered.)