The charge is more than some cheap OTC medicine you might get prescribed, e.g. hay fever tablets are pennies per box and the charge is almost £10. But pharmacists are required to tell you if you could just buy it yourself for less - they collect the charge so they have the opportunity to point this out whereas your doctor needn't know if you pay or not.
Logically the charge must also be bigger than the price for some of the cheapest prescription-only medicine and there's no way to avoid paying extra in the UK as far as I know in this case.