Britain shouldn't be in Afghanistan. But, since we're there, let's rethink the whole policy on opium farming. Here's the deal.
We should let the farmers plant whatever crop is most suitable for their environment, even if that's poppies. The central government should collect all the opium harvested, and pay the farmers more than they would receive from the smugglers. The world's requirements for morphine and other opium-based products could be met by Afghanistan. The exports would help offset the cost of paying the farmers well enough, and the international community could contribute funds to make the operation financially viable.
I know there must be good reasons why this isn't being done, but I don't what they are.