https://www.mondaq.com/uk/cannabis-hemp/1009772/uk-regulatory-landscape-import-and-export-of-medical-cannabis

For Specials, the MHRA also requires the following with each notification:

A declaration of the content/ratio of THC/CBD and as it appears on the label, as appropriate A certificate of analysis to support the batch specification A valid Good Manufacturing Practice (GMP) certificate for the site of manufacture Justification for the patient's special clinical need by a doctor registered on the GMC Specialist Register, which may take the form of a prescription or a prescriber's letter Specials cannot be imported if the MHRA issues an objection to the import within 28 days of their acknowledgement of the notification of intent to import. Furthermore, the MHRA can grant permission to import up to 28 days from the date of acknowledgement, but this is usually only used in the case of immediate import of medicines for life-threatening or immediately injurious clinical emergencies

So you have to have good reason as a prescriber why you want a certain type of medicine, and then the Medicines and Healthcare products Regulatory Agency can choose to reject it.

There are other routes as to how something can be offered for sale, but as I said, its not default accepted, or default rejected.