Can has been open for years.
Older antihistamines that cause significant drowsiness (classically Piriton or Chlorphenamine (generic name for Piriton)) are incompatible with solo pilot operation.
Newer antihistamines MAY be acceptable but probably only after a period of use with a safety pilot to demonstrate that drowsiness is not an issue. I know many people who get drowsy with the so-called non-sedating antihistamines and so they would not be safe to fly solo.
At one point the RAF only approved one non-sedating antihistamine for flight use. Unfortunately it has now been withdrawn from the market due to other problems. I am out of the Occ Med loop at the moment so I don't know what the current mil advice is but I would expect the CAA advice on potentially sedative drugs to be similar.
Beware that the longer acting (once-a-day) drugs take longer to leave your system so the length of time after taking a drug that you should not fly will get commensurately longer
Hope that helps but I suspect it just stirs the already muddy water!
Bob