The Drugs Act 2005 regulates in Part 1 the supply of controlled drugs. Part 2 describes the police power related to drugs and in Part 3 the assessment of misuse of drugs is regulated.
Finally, in Part 4 (miscellaneous topics) the mushrooms containing psilocin are defined as Class A drug. Furthermore, amendments and repeals are generally regulated. Schedule 1 describes the amendments in detail, like schedule 2 does for the repeals. Full text see Drugs Act 2005