I'm not sure you can read anything into the report that, since it's downgrading as a drug, cannabis use has gone down.
If you were of a particular bent you might argue that it suggests that if you treat people like adults then they tend to act all adult-like.
Or, though maybe not mutually exclusively, you could argue that the less "criminal" a drug is the less lustre it has for certain people.
One might also note that decriminalising the weed has not sent the youth of Britain spiraling into reefer madness. Far, it seems, from it.
It's probably nothing to do with any kind of legislation, I mean, the youth of Britain spend some much time binge drinking it's a wonder they find time to do any drugs at all.