15 and can't afford a PPL
The Air Cadets as above is a good idea, Her Majesty has got loads of aeroplanes.
Also, why not think about gliding. Sure, it's not for me, I like to take at least one engine with me each time I get airborne but many don't.
It's cheap and I'm sure it'll teach you a lot that you couldn't get elsewhere.
You can always then get a NPPL (SLMG) quite cheaply, swap that for an NPPL (SSEA) and you've got a PPL.
Another tip form an oldie - enjoy the journey, don't worry about where you're going (that's a general life tip and not necessarily a Nav tip).