On minor point

Until they test the substance, how do they know it's not illegal to possess?

I agree with the tone of the article, the explicit threat from the police is very worrying and something anyone with a brain could have seen coming. I can't help but think that the police are caught in a legal though (perhaps deliberately).

If they let people out whilst the substances are tested, there will be stories all over the press about drug-dealers being set free.

If the police take a harder stance, you get stories like this.

I am not not sure what the actual answer is (and I doubt we'll get one from Labour or the Tories), but continually kicking plod doesn't strike me as being it.