By law, doctors can only prescribe a 30-day supply of medical marijuana which can technically be provided by any pharmacy. A prescription will state the type of marijuana medicine (oil, flower, capsules), how much THC or CBD is contained, the dose, and how often it should be taken.
Any CBPM supplied to patients in the UK must be approved and licensed by the Medical Healthcare Agency (MHRA) and Home Office, meeting strict criteria such as Good Manufacturing Practice (GMP).
The reality is that, as CBPMs are unlicensed, not all pharmacies will be able to supply the medicine, so you may need to approach various before finding success. According to the Centre of Medical Cannabis, ordering the medicine will take between two days to two weeks.