Hi Oliver,
Nothing about flying is cheap but it's great to see your enthusiasm. I share it and still get excited everytime I get in a plane as pilot or passenger (and I'm just a lowly PPL). Gliding is one way of getting some cheap(er) flying experience and is a great way to meet people who are in the field. Also the Air Cadets do flights for cadets a few times a year in a Grob Tutor. Unfortunately the glider fleet is currently grounded but may well be up and running again next year.
The cheapest way of learning to fly is to get the RAF to pay for you but this isn't an easy route, isn't the right route for everyone and requires a lengthy commitment (not sure how long). The second cheapest is to get on an airline sponsored scheme such as the BA Future Pilot Program. Again as you can imagine the competition for these places are incredibly high (70ish taken from a pool of thousands).
There are pages of debate on here about the differences between integrated (eg CTC Wings - 1 long 15ish month full time course) and modular (eg. your local flying school - probably a minimum of 2 years part time), but as your question refers to little money, a modular route costs less than an integrated route.
Finally as redsnail says - get your vision (and hearing) comprehensively checked - most opticians do free eye tests for children. I found out at age 13 that I was colour blind - and my mum isn't either! - which destroyed my hopes of going into the RAF. The colour blindness gene is carried by our lovely mums but they very rarely suffer from it. Fortunately a new test was devised a few years ago in conjunction with the CAA called the CAD test (
https://www.youtube.com/watch?v=MOJpZMors7A). As a result of passing this test I have been able to obtain a class 1 medical, so even if you fail the Ishihara (dots showing a number in different colours -
https://www.youtube.com/watch?v=sB28zwvY4Zg)
Good luck and happy landings.