For the overwhelming majority who take illegal drugs, drugs laws lack credibility and the way they are enforced is inconsistent and confusing, making them an ineffective barrier. There is tension between politics and science, which draws the drugs classification system into disrepute. Politicians do not want to "send the wrong signal" about how harmful drugs are and so resist reclassification of drugs downwards whilst scientists want to be objective about relative harm, even if it means downgrading a drug. As the drug classification system is a fundamental part of UK drugs laws, this brings the law as a whole into disrepute.
When I was a student I thought cannabis was a harmless pastime, no worse than alcohol, and I was encouraged by the fact that it's not addictive. Many years later I ended up working for an addiction rehab clinic in Scotland where I found out that not only is cannabis addictive but the skunk version of the drug is known to send as many as 20% of users into a psychotic state.