The Psychoactive Substances Act took effect in the UK in May 2016. The Act makes it an offense to produce, supply, or offer to supply any psychoactive substance if the substance is likely to be used for its psychoactive effects. The only exemption from the Act are substances that are already controlled by the Misuse of Drugs Act. Essentially, any substance will be illegal to produce or supply if it is likely to be used for psychoactive purposes.
Possession with intent to supply as well as importing or exporting a psychoactive substance will all become offenses even if the substance is purchased from a non-UK based website.