Perhaps the issue should be tackled at the 'grass roots' first. This is quite a useful website:
http://www.drugscope.org.uk/resources/faqs/faqpages/where-do-drugs-come-from
More emphasis and resources placed upon tackling the import and UK production of the actual drugs themselves would be the ideal resolution if the government threw all the money they could at it. Without the drug in existence, the dealers and users would have no source.
Education in schools is underrated - More should be done to ensure that youngsters don't get hooked in the first place and if they find themselves addicted, they should be aware that there is help out there to get off the drugs rather than the ever lasting spiral that some drug addicts fall into, and especially HOW to seek help. Most addicts feel helpless and ashamed and continue to use as they feel it is the only way to function - I know support for addicts costs the NHS, but the fact remains that so many are already are addicted and this cannot just be swept under the carpet with a quick fix solution.
Stop the source, educate more, and help those who need it. If we had the answers already wouldn't that be good, but we don't, not just yet and illegal drug use in this country won't just disappear overnight.