How true the claims are is unclear. What is clear, if you go to the MoD website, is that they have used the "UK street value" of drugs that are in Afghanistan.
So there are two fibs for the price of one from defence secretary John Hutton:
"The seizure of £50m worth of narcotics will starve the Taliban of crucial funding and prevent the proliferation of drugs and terror on the UK’s streets."Obviously the Taliban were not going to get anything like £50m for the drugs while they were in Afghanistan. Neither will the seizure of drugs that were by no means destined for the UK prevent the proliferation" of either drugs or terror here.
The Independent's very dramatic piece on this looks like it was written by the MoD press office.