The key is in the first sentence of the article: 'The proportion of adults who say they use illegal drugs has fallen to its lowest level since records began, a Home Office study suggests'.
I'm sure that many of those adults who once would have taken illegal drugs, are now using 'legal highs' instead in order to avoid problems at work due to the increasing likelihood of company drug testing and to avoid getting a criminal record if caught. So while our government and law enforcement are patting each others' backs and sucking each others' cocks thinking that they are winning the 'war on drugs'; in reality, increasing numbers of adults are risking their health and sanity on more & more untried & untested compounds. Why we have the ACMD is a complete mystery to me, because the government never listen to a word they say (e.g. the recent ban on Khat).