this really belongs on the Medical forum but here is my answer anyhoo.
Im 24 and have asthma like forever. Was hospitalised over 10 years ago so no limitation there as the rules are you must not have been hospitalised within the last 10 years.
Also, using a preventative inhaler (becotide) and the reliver (ventolin) is no problem. When i went to Gatwick, they tested my max lung capacity. Then how much air I can expire in one second. This is the FEV/FEV1 ration. As long as this is over 70%, then no problem.
However!!!....
They make you go for a 6 minute run and straight away to the test. You must still reach the 70% ratio. Then they make you relax for 10 minutes then do the test again.
What the medical people want to see is a no appreciable drop in the ratio. Thats all I can say. I did a hell of a lot of running and swimming for 4 weeks before the test, and I really got fit out of it. I even lost 4kg!!! The guy said I had one of the largest lung capacity results he has ever seen and the doctor said there was no problem when my ratio was just 71%. Apparently, the larger the lungs, the more difficult it is to expel all of it in the first second - obviously. Anyway - I passed. The only limitation I have is I need to get myself a pair of specs. Eyesight not to good in one eye.