One thing is for sure - you'll never get an answer from NATS before you go through the mill of the selection.
I would say play it down - say, as "slight tendency..." or similar. Only you know how much trouble it is or might be. Unfortunately - and obviously you are a typical example - doctors will immediately classify you as "asthmatic" for the slightest symptom and there it is on your medical record, which can then screw you for possible employment in later life. On the other hand, if stress or similar can bring on an attack, perhaps do yourself a favour and avoid any occupation where it would be an embarrassment.