From a Guardian piece, "Cannabis use down since legal change":
Gordon Brown's plans to tighten the law on cannabis by increasing the penalties for possession suffered a fresh blow yesterday as the latest official figures showed the decision to downgrade the drug had been followed by a significant fall in its use.
Since cannabis was downgraded the proportion of young people using it has fallen each year from 25.3% in 2003-4 to 20.9% now. Among those aged 16 to 59, the proportion over the same period has fallen from 10.8% to 8.2%.
Claims that "superskunk", which is up to 10 times stronger than imported resin, was dominant were also undermined by police and customs seizure figures. The latest statistics published yesterday showed that 69 tonnes of cannabis were seized in 2005 of which 49 tonnes were the traditional resin. Although herbal cannabis seizures, of which "superskunk" is one variety, were up by 76% - reflecting the growth of a homegrown cannabis industry - they still only amounted to 20 tonnes, less than one-third of the market.