In 2018, the UK amended the Misuse of Drugs Regulations 2001—not the 1971...

https://mike-wiki.win/index.php/How_Can_Cannabis_Be_Class_B_but_Schedule_2_at_the_Same_Time%3F

In 2018, the UK amended the Misuse of Drugs Regulations 2001—not the 1971 Act—to allow certain cannabis-based medicines like Sativex and Epidyolex by prescription. These remain Class B drugs, prescribed only by specialist doctors under strict rules