There are a couple of problems that need to be addressed before it can be done. Firstly, it is not curing an addiction or fixing the problem. It is feeding the addiction and masking the problem.
The problem still remains that they are addicted to heroin. How long before the amount given on the NHS isn't enough? Before they're forced by their addiction to find a dealer to buy it off?
Secondly, where will they store the large quantities of heroin? In Boots?
If it's stored in a pharmacy, there's a good chance that the places will be ransacked by addicts who want more than they can get, or by dealers looking to get some free narcotics.
I personally wouldn't store it in hospitals either. Casualty on a Friday night is bad enough without heroin addicts demanding drugs. Especially since the drunks that are already there are a nuisance at the best of times.
In my honest opinion, the plan is unworkable. Not only that, but it's one step towards legalising a narcotic (and probably the worst one).
I do have sympathy for addicts, but if I'm perfectly honest, I'd rather see the money being spent to help others. Perhaps get more effective cancer treatments etc. Since we already have pretty good support structures in place for heroin addicts to be weined off of the drugs.