Whoa there,T2 ! That's not quite what I said!
Asthma needs to be diagnosed by a specialist. If it hasn't been diagnosed you don't yet have it. You need to know if your GP has written "query? asthma" in your Notes. If she has, you may (v e r y politely!) ask her how she would feel about altering it to something less specific until such time as it has been diagnosed by a specialist, and explain that the reference may compomise a job application. She will know nothing about flying, but she will have come across the same problem before with other occupations, HGV, police, firemen etc.
Forget the chocs and daisies and don't even think of a chat-up, but a very nice smile will go a long way.
It's a (legal and very reasonable) r e q u e s t, remember, and a chance to see and discuss if there's anything else in your medical notes that may be a problem in your application.