I agree with the above comments about doing it nearly full time. Everything I've done from driving lessons, boating and flying has been full time and the reason I think I picked it all up quickly was thanks to being full time (and young!). Having said that, I know how itching you are to learn to fly. If you can hold on, do. If you can't wait that long, we all understand why.
You could do all your theory exams for the PPL but I would have got completely bored and demotivated without flying or being in an aviation environment. When doing my PPL I flew at least 2 hours a day and after my lessons I was on such a buzz I studied hard.
As for answering your question, if you want to build experience to get hours under your belt there's a lot you can do but it's all very hard to get the opportunity to do it. There's instructing, bush flying, glider/banner towing and probably some others I can't think of right now.
Regarding qualifications, get as many as you can and do as well as you can. I wish I did. Most will only require 5 GCSEs A-C but don't rest at the thought that you've met their minimum requirements. I don't have any experience with qualifications and how airlines perceive them though.
Best of luck!