Hi there Irish wings
I too have hayfever and am currently studying to become an airline pilot. I would recommend that you contact the CAA at Gatwick or your local AME.
I currently take Loretadine/Clarityn, when i contacted the CAA a few years back they also allow the use of Telfast, although i would give them a call. As far as i'm aware they do not permit any other anti hystanmines due to their drowsy side effects.
Hope this helps
Regards
Bozzato