Sorry, I wasnt clear. The £24 a year is what you pay to PCUK to be a member of pony club, you then pay your RS for the PC sessions which usually include a group lesson and a classroom/yard based session.
You can be a member of pony club until age 26 and, if not the acheivement badges, the tests go to quite a high standard. The A test is equivalent to BHS stage 4 (which, if my research is correct, is actually higher than one of the instructors qualifications!) So it's not all geared around little kids.
My old RS ran a pony club session right before my weekly lesson, and while there were younger kids there, there were also teenagers and all looked like they were enjoying it.
You'll probably have to phone them up to find out the price, but as Faye said, it's often cheaper than your normal riding lesson. Plus, at my old RS, pony club members, got a discount on any extra lessons too!
Please talk to your instructor before you go for a part loan or share - if he or she doesn't know of one available, they will at least be able to advise you on what to look for and what to avoid.
While you're talking to your instructor, ask if there's any possibility of helping at the stables on a Saturday or Sunday - its something of an equestrian tradition; teenage girls helping at the stables at the weekend! (Its how horseless kids learned before pony club centres came along)
Please excuse my spelling/grammar mistakes, I'm British.