Go to the RAF and find out now. Don't spend the next 7-8 years working towards it having set your heart on being a pilot only to find out you can't do it.
Basically though if you have asthma you can't fly planes for the RAF. Not sure on the situation for commercial flying (airlines etc)..... generally the mediacal standards arn't as tough. I didn't get into the RAF as flight crew because of colour vision deficiency but I hold a JAA class 1 (which means I can fly commercially).