Since 2018, medical cannabis has been legal in the UK for certain conditions....

https://post-wiki.win/index.php/Does_Marathon_Training_Count_as_a_Medical_Reason_for_a_Prescription%3F_A_Reality_Check_for_Runners

Since 2018, medical cannabis has been legal in the UK for certain conditions. For runners, access is facilitated via specialist private clinics. Generally, patients must have trialled two conventional treatments before becoming eligible