Defra/VMD: accredited internet retailers authorised to sell veterinary medicines
Since 2005, you are allowed to ask your vet for a prescription for medication. You then upload or email to the online retailer to purchase the drug.
Veterinary prescriptions are priced by the practice and from experience can range between £6 to £40 per prescription per drug. I paid £15.50 (Summer 2019) but six miles down the road another practice charges £40. Do your homework when registering with a practice.