I also suffer from asthma and went over a year ago to get my class 1. What the make you do is blow really hard (ooh err) to check the lungs pulmonary function or something, and then sadistically make you run round the building 5 times till your totally shagged and measure you lung function again.
If your in reasonable health and don't really on the blue inhalers much, you shouldn't have too much trouble passing the guidelines for asthma. However, if you don't meet them, they recommend that you see a specialist who can help your asthma more than a local GP, and if you show improvement to above the standards, then the CAA may accept the specialists report.
Basically, get running and get fit before you go. Increase you lung capacity before the test and you should have no trouble.
PS. I hold a class 1 so I know it can be done.