I get horrid hayfever and had a hard time when I was pregnant and BFing.  You can get things from the homeopathic but I didn't find them any good. 

There were a couple of things I used while I was pregnant that did work, both are available from the chemist.

The first (and prob the best) was a sinus rinse.  It's basically a bottle in which you mix up a saline solution (packets come with the sinus rinse kit) then you squirt it through your nostrils to clear out any allergens that you've breathed in.  It sounds and looks worse than it is, it's actually pretty easy to use and really clears the sinuses.  You can use it once or twice per day.  I get really bad hayfever so I was using the strongest saline solution twice per day.

I also had a gel I think it was called hayfever med (not 100% sure though, it's a little white tube with green writing and a green first aid cross on it).  It's a completely natural formula which you rub into the nostril and it prevents you from breathing in allergens.

I still had trouble going places where my hayfever flares up (like my parents house in the country) but I have trouble there during hayfever months anyway, even when I take antihistamines.  So I avoided the worse places for my hayfever and used the sinus rinse and hayfever gel to keep my symptoms at bay.