If you have hay fever on your medical records, you will NOT be working as RAF aircrew - full stop. If you suffer from a summer cold, shut the f up about it and go for the career choice that you want - it's an easy choice.
Again, in case that was not clear enough - you will not be allowed to serve as RAF Aircrew if you have a medical condition that has been diagnosed as hay fever. On the other hand - do not self diagnose, you wouldn't wander in to the AFCO and say "Oh! I also think I have might have cancer, asthma, diabetes and no spleen, but will you still consider me?"