This is a late reply, but Google sparked it up, so I figured I would give some insight.
When I went to RAF Careers, they asked me a set of general questions... Is your eyesight okay? Do you have asthma? Do you have diabetes? Do you have hayfever?
The last one struck me too. I replied, honestly, and said... Yeah, I have very mild hayfever.
My hayfever, as a child, was quite bad. But I have learnt to deal with it, and my body has responded. Nowadays, my hayfever is not a problem.
The RAF Careers Sergeant replied... Do you take any medication for it? Once again, I made an honest reply and said no. The matter was not taken any further. I have heard all sorts of weird reasons why hayfever is a concern. Some people say it is a personal weakness, and that the military don't want weak candidates. Others say the the oxygen masks will causes a reaction to your skin (sounds like crap to me).
So long as the hayfever is mild, I wouldn't worry about it. Learn to take yourself off the medication, and I don't think they will have any problems with it. I don't know why it's not allowed, but whatever keeps them happy. It could even develop with you as an adult, so it's not dreadful. It's not like having a missing limb or anything. My greatest concern (which I probably mistake for hayfever) is known as ACHOO syndrome... I sneeze in sunlight. It affects quite a lot of people, it just means that my body dislikes the bright light and tries to make me close my eyes. At least, this is my excuse if they challenge my summer sneezing... And it's only a white lie!
As for Air Cadets... Join if you can, or at an elder age, apply as an adult instructor. It will give you an insight into RAF procedures. It will teach you discipline, it will teach you to dress like an RAF candidate and it will teach you the basics. But beyond the boring stuff, it will be great fun too. You will learn to fly (with scholarships available, as well as many flight and glider opportunities). You get to learn to shoot, you get to learn to camp and survive. It's just a brilliant personality building block. You will meet a lot of new people, and you'll be able to make a name for yourself. If you stick with it long enough, you'll probably be able to get a good reputation with a wing commander too... And if you asked them for a reccomendation, I am sure they will be able to help with any career path you try to choose.