In exciting news for capital-dwellers, the ‘war on drugs’ in London is about to become even more woefully incoherent than it is already. Sadiq Khan has apparently announced plans to “begin decriminalising” drugs in the capital.

Of course, the Mayor doesn’t have the power to actually decriminalise anything. What this appears to be, according to press reports, is instructions to the Metropolitan Police being told not to arrest “young Londoners” caught in possession of Class B substances. They will instead be directed to a counselling scheme.

For those of you unfamiliar with the UK’s mostly arbitrary drug classification system, here’s the somewhat breathless summary from the Daily Express: “cannabis, ketamine and even speed”. (For those unfamiliar with said drugs, if the word ‘even’ has any place in that sentence it’s in front of ketamine.)

This has obviously put Khan on a collision course with the Government, which has been making a lot of noise in recent weeks about a crackdown on ‘middle-class drug use’. But it also seems at odds with the Met, which in recent days has faced a backlash over a high-profile stunt featuring officers swabbing people for Class A drugs outside venues in Shoreditch.

Ah, you might think, but surely Class A drugs are much more serious? But don’t be fooled: Class A is a nonsense category that compasses things such as heroin and crack, which in public health terms are very serious indeed, and MDMA (ecstasy) and LSD (acid), which in the same terms are complete non-events, posing little-to-no danger to users or the people around them.

For its part, the Met justified the move as part of “a ‘week of action’ supporting women’s safety”; the only person they arrested was, inevitably, a woman.

Nor is the connection between the two issues obvious anyway. There may be a link between Class As (which?) and crime, but do they play a larger role in assaults on women than say, alcohol? Are the calculating predators spiking drinks also getting themselves high on cocaine? It seems a stretch.

Regardless, the point is that the clear boundaries between different classes of drug do not really extend beyond the heads of policymakers and the police.

The black market seems to have naturally sorted itself along more rational lines: weed is often sold by dedicated dealers, as is the really monstrous stuff such as heroin. That means the more common Class A substances such as coke and ecstasy are sold by the same dealers who peddle Class B alternatives such as ket or mephedrone.

As a result, the counter-signal Khan is sending by pushing to decriminalise the latter will almost certainly do more for sales of the former than the occasional show of force by the police will hurt them.

Which in turn points to the underlying conceptual problem with ‘decriminalisation’ as a middle ground between actual legalisation and effective prohibition: it lifts any suppressive effect there might have been on demand whilst leaving supply, and thus the profits, entirely in criminal hands.

This doesn’t mean that trying to prosecute people for possession of Class B substances is a good use of police time for a force that has a woeful record on more serious crimes. In a world where legalisation is not on the table, policymakers can only try to choose the best of a range of bad choices. But even the limited gains of such an approach will be lost if the Met simply divert any resources spared into a showy but futile campaign against other drugs being sold by the same people, to the same people.

Ultimately, the best results from this sort of approach can only follow a proper overhaul of the categorisation system, based on a fresh assessment of each substance according to clear public health criteria rather than historical accident and inherited prejudice. But that is in the gift of the Government, not the Mayor.