I'm glad you gave us an update! I just wanted to let you know that my son, almost 2 w/ CF) developed this dry, repetitive cough early this summer. I could hear a wheezing noise from across the room, especially after he ran around a bit. Sometimes he'd cough at night. Sometimes he'd cough til he gagged and then he'd vomit. Other than that he was fine, not cranky, eating well, sleeping well. I was afraid he had a lung infection, but the CF doctor told me the same thing they told you. It wasn't a wheeze in his lungs, it was his more like in the back of his throat and you could hear it rattling. It was post nasal drip, caused by allergies. It was also irritating his throat and making him do that dry cough, and clearing his throat. And when he got coughing the post nasal drip would make him gag. They put him on an allergy med (Extendryl) and he did fine. They took him off it this fall, and he's doing well. It must have been a seasonal allergy, I'm sure we'll notice it start up again in late spring.
What you described sounds similar, even though my son does have CF. Have the doctors tried something for allergies? Maybe there's an allergen bugging him where you live that's not in the UK, causing post nasal drip?
I'm glad to hear he's doing so well weight wise. I wonder why they take so long to give you the results? The sweat test results should be back same or next day. Bloodwork for genetic testing would take weeks though. I hope everything goes well, and that its not CF, and that you can get some answers. Keep us posted!