Dmackie.
It certainly didn't stop me getting in the RNR (I was checked out by regular Medical Staff) I assume it wouldn't matter for the Fleet air arm. The problem with Hayfever is that it can be a trigger to Asthma so my background was checked (Its given me chest infections but not asthma)
My father told me a story about Culdrose circa 1974. The Pollen count was so high that guys that usualy didn't suffer ended up at the sickbay with all the usual symptoms (Some even had breathing difficulties) It was so bad that the MO just left the issue of antihithstamine to the Duty Sick bay Attendant.