You will need to pay the Pharmacist the Prescription Charge (currently £9.35) for each individual item (ie a pair of support stockings will be £18.70), unless you are exempt from charges.
A pre-payment certificate may be cheaper if you are likely to require more than 12 individual items over a 12 month period (it currently costs £108.10, or for 3 months, £30.25).
Many people do not have to pay prescription charges e.g. due to age, certain benefits, pregnancy, certain medical conditions, for medicines received at hospital or walk-in centres etc. For full information on prescription charges and all the different exemption categories (it’s very complex!): NHS prescription charges