As a rule, pharmaceutical imports into the UK are duty-free. Remember that in any case, duty is assessed based upon the value declared for customs purposes on your shipping forms. If your mom declares a low value, even if duty is payable, it would be minimal. It's always good to enclose a photocopy of the doctor's prescription for the medication in case there's any question. (I work for a shipping company that specializes in servicing the pharmaceutical industry.)