It's just a question of time until SN will be forbidden. When do you thing will SN be as difficult as N to obtain? Months, a few years?
We can only speculate on what action the UK authorities have taken, but certainly something seems to have been done to severely restrict the availability of SN to those who might be inclined to misuse it (as they would see it.) Legitimate users still have licence free access, so it looks like the supply regime is now considered satisfactory. There is so little profit to be made from selling the comparatively small quantities required that few, if any, suppliers will see any point to taking risks. There will never be the demand for suicidal SN that there is for recreational narcotics, so the limited market will not grow and so there is nothing that can be exploited for big bucks. I imagine that small quantity suppliers in UK have had a knock on the door, and a word in the ear that they are being watched.
SN could be transferred to the regulated category (i.e. requiring a licence) quite easily, using secondary legislation, as there is a so-called "Henry VIII clause" in the 1972 Act. This is a device which allows the relevant Government minister to add substances to the regulated category with little difficulty (see section 2A of the 1972 Act (as amended). A document called a Statutory Instrument is issued, and whilst opportunity to debate this in Parliament is usually given, there is no need to follow the usual lengthy procedures of changing the law. However, this doesn't seem to be necessary as legitimate supply has been effectively interrupted by easier means.
Whilst there has been an increase in the rate of suicide in UK for the last reporting period, much of this will be a statistical blip due to the change in rules which allow Coroners to decide on the basis of balance of probabilities rather than beyond reasonable doubt. Even so, there are less than 20 suicides per day in UK. Given that about 50% of these are by hanging, and maybe another 25% by other methods, then there's only going to be about 5 people per day who suicide by poison, at a very rough guess. Of course, many more will attempt, and fail, but even if this is ten times the success rate, there is no worthwhile demand to encourage suppliers to take any sort of risk in making SN available.
I think that we have seen the effective end of SN as a method of suicide in UK. There will always be the odd case, and people will acquire it in one way or the other, but the numbers will be small. At present, I expect quite a few people will be sitting on stashes acquired when it was easier to buy, so there may be cases caused by these caches. However, we do not hear about them all, as part of the anti-suicide strategy in the country involves persuading the press to keep quiet about exact means being used.