I'm wondering if the only reason it hasn't been banned is they think it's an unpleasant enough of a death for most people to avoid it??
A SN death should not be a painful or particularly unpleasant one. Especially not if you plan, prepare and research facilitating it.
Yeh, like happened here:
https://sanctioned-suicide.net/threads/goodbye-thread-today-10-00-pm-utc-2.50562/page-4
Trayus here took only SN, nothing else. Had they followed the PPHB or Stan's guide, their death would likely have been a more peaceful one.
How long til it'll probably be banned in the UK?
As for your question, nobody knows how long till it gets banned. But this is the UK, so I'm definitely expecting it to at some point, or at least for it to be HEAVILY HEAVILY restricted.
I don't think SN will suddenly drop off the market i.e. one day you can buy it, the next you can't. Rather it will just become increasingly, increasingly, increasingly difficult to source over a period of time, if laws are introduced.
Some people think it will never be banned, because of its use in curing meats and such - SN is very useful. This is why (as you asked), it is "so easy to get".