I have been wondering about this myself. Over the years I have imported various things, mostly vitamins from the USA, and I've noticed that sometimes I would get a VAT bill and sometimes not, for the very same goods. On that basis I surmise that UK Customs stop random packages and examine them for VAT eligibility. Of course the higher the declared cost of the goods, the more likely to be stopped. Those Customs guys still need to get their salaries paid. Therefore I'd say that anybody importing meds needs to budget for UK VAT, although they might get lucky on it.