The report sent to the then home secretary by the former chair of the Advisory Council for the Misuse of Drugs highlighted a number of serious concerns around criminalising drug use. It is the only ACMD report not to have been published.
It said there was “little consistent international evidence that the criminalisation for possession of drugs for personal use is effective in reducing drug use”, that the UK was not required to criminalise drug use under its treaty obligations, and that criminalisation harmed people’s educational and employment prospects.
I think it’s common knowledge that they recommended this back in about 2009, and I think the government notoriously sacked their own scientific advisor rather than listen to reason, perhaps it’s a new leak that it was recommended again in 2016? (Although not at all surprising that they did so!)