There re some Heroin programs in the German speaking word for quite some time now, especially in Switzerland and Germany. In Germany you can choose your dose freely, but you have to be able to shoot up yourself without any help from anyone else and you MUST shoot up. In Switzerland, for example, you're allowed to smoke it, too. Anyway, the highest doses reported in Germany were 750 mg pure Diamorphine per injections, though people are allowed to do more. I guess there is some kind of ceiling effect. The downside is that, at least in Germany, you MUST shoot up under sight. You can never ever get a take home, though you are allowed to combine your Heroin prescription with a Methadone prescription, which you are allowed to take home for 1 or 2 weeks worth of Methadone at a time. You can, in most regions of Germany shoot up Drugs you've brought yourself under sight, too and thus reducing fatalities. Also you will be provided with most things you'll need on entering the facility such as clean syringes and needles, disinfected spoons, acid in water fit for IV. The only thing you can't get are sterile membranes for micro-filtering. They will give you cigarette filters instead. Police is not allowed to enter these facilities, but might stop people and check ID if your hanging around by the entrance too long.
Munich is a little backward in all these things and has many more deaths due to overdose. What has been observed in Germany is, that the ore relaxed restrictions are, the less people die of drug-related illness, especially overdoses, but infectious deceases, too.
In Germany many people leave substitution programs (don't get new prescriptions), if the street heroins purity improves to about 60% r higher. But this happens rarely and arrests of dealers are made fairly quick in these cases.
In the UK you can get Heroin on a regular prescription, or at least you used to. Don't know if it is still like that, however I think the hurdle is much too high as you have to fail a regular replacement-drug therapy with Methadone or Buprenorphine like a dozen times... But at least you can get methadone to IV in UK, too. In Germany this might only be possible if you're getting Methadone for pain.
If you ask me I'd say that maintenance therapies should be conducted just like opioid-supported chronic pain treatment: an opioid with long duration of action and for acute ad strong cravings another instant-release opioid with short action and short half-life. Opioid rotation as necessary. But as long as these evil "Junkies" are not seen as people who are ill and need medical help and rather as something the "normal population" needs to be protected from as they spread desease and financial hardship due to their criminal behavior, nothing much will change, unfortunately, and drug-addicts will always be considered unworthy sub-humans.