Advice re Hayfever

Hi all, over the last few years I have developed what I think is hayfever or the like, and it's getting worse and worse. I've got a dry hacking cough, runny nose, runny eyes etc. I sometimes cough so badly I'm sick. My chest is clear and the GP just gives me antihistamines which do not appear to be doing anything at all. Allergy testing on the NHS is no longer available so I've bitten the bullet and just paid £158 and had tests for inhalants at my local private hospital and am awaiting the results. I find if I am in a room where the windows are open, I can't stop coughing, and of course this exacerbates my stress incontinence. I've looked up "Air Purifiers" as I wonder if that is the next course of action? They seem to vary from as little as £35.99 right up to £400. Has anyone ever bought one and if so, which one and does it help? I'd be really grateful for some advice. I'm at my wit's end.