Not if it's on a suicide mission. Russian subs are always in sneaking into UK waters. It would be quite easy for one of them to attack us if it wanted to. Whats stopping a NK submarine doing the same?
Russian subs have a far higher level of stealth capability - NK subs have pretty much none - even switched to electric power they have very limited range/performance and are very very detectable.
EDIT: NK developing SLBM platforms is a very real danger but its quite a lot longer down the road than their other advances.