I agree totally with Mach086.
I had the same problem (and posted in some detail what to expect on the medical forum).
Im not a doctor but I think that through excercise you can train your body to use less oxygen the next time you excercise.
The CAA use running as a test because excercise (in particular the deep breathing) will bring on your Asthma. So if you have to breath less for a set amount of excercise then your Asthma symptoms will be less.
These are my own theories and not medically approved, recommended etc. But that Mach086 and myself did seemed to do the trick.
Id recommend doing a lot of running and getting yourself a peak flow meter from Ebay so you can test yourself before and after your running.