Controlled under the Psychoactive Substances Act 2016. Under the Act, possession for personal use is not an offence, but the following activities are:
Production (including for personal use);
Supply or offering to supply;
Aggravated supply or offering to supply (near schools, prisons etc. or using someone under 18 as a courier);
Possession with intent to supply;
Importation;
Exportation;
Possession in prisons (and other custodial institutions).
More detailed information on the Act can be found here.
For all offences, the prosecution will have to prove that a substance is psychoactive. Because this is still a relatively new law it is hard to say what will happen when cases come to Court.