I heard the strangest of tales last night about someone who had a cold and went to a major high street chemist. They tried to buy two packets of Lemsip over the counter and were told - in no uncertain terms - that they were not allowed to purchase two packets of decongestant analgesic because of the law.
Now there are certainly restrictions on the amount of analgesics (e.g. paracetamol, aspirin and ibuprofen) that can exist in a pack. And some stores do have voluntary policy limits. But as far as I can tell there is no law that states you cannot buy more than X at any one time.