Legalisation would enable market regulation
In the absence of regulation illegal drugs, unlike alcohol and tobacco, cause even more harm than they would if they were regulated.
The market in alcoholic drinks is heavily regulated. Alcohol cannot legally be sold to minors or to people who are clearly drunk. There are limits on the amount of alcohol that alcoholic beverages can contain, and the amount of alcohol that they do contain must clearly be indicated on the bottle or can. Alcohol beverages must not contain contaminants or other illegal substances.
In the case of tobacco, many of the same stringent requirements pertain, and smoking in public places is already banned in the UK or shortly will be. All cigarette packages have stringent health warning printed on them.
The upshot of such regulation is that, while alcohol and tobacco cause great harm, they do not cause nearly as much harm as they would if they were wholly unregulated.