Since OP already mentioned he has managed to acquire SN (and since he lives in the UK where acquiring a gun isn't all that easy, I believe), I'm assuming the "execution" will take place in the form of an overdose on a toxic substance, rather than a shotgun blast. There are plenty of different ways an actual execution can take place (including a lethal injection, which would probably be the closest equivalent to OP's plan), so no reason to assume a gunshot is the only way forward.
I imagine it feels relieving to know one has a surefire way (as sure as something like this can ever be, anyway) to end things at any point. If I had the means, I'd probably go the poisoning/OD way, as well. Seems like a relatively easy and painless method that can reliably provide the desired result, as long as you've got the right stuff and enough of it. Even if you're 100% certain of CTBing at some point, having a method at your disposal and the ability to fall back on it at a moment's notice will surely make it easier to go on a while longer.