The prisons are full of drug dealers and addicts which costs the country untold millions. Give them a legitimate way to contribute and save the country a fortune. Those that will abuse drugs will do so whether they are legal or not so why not tax them and let them contribute to their country's upkeep. Plus the prisons then have room to house law breakers that are a danger to the general public (rather than just those that are a danger to drug addicts).
Legalise drugs and tax them like alcohol and cigarettes. Anyone that needs drug related medical treatment has contributed to the cost via the tax they have paid. Those that can enjoy drugs responsibly contribute to the rebuilding of the UK economy legitimately. Afghanistan also has a legitimate outlet for it's most significant export, helping to bring the country into the 21st century.