I've never fully understood the logic (even though, unusually, it's actually stated!) of BS7671 about this. In the absence of faults, there obviously ought never be a dangerous voltage between N and E within an installation. However, if one is going to consider the possibility of faults, then I would have thought the the only (almost) 'totally safe' situation for SP isolation would be with TN-C-S, since the N and E are actually joined within the installation. However, BS7671 seems to think that TN-S is also OK for SP isolation (which it will be, unless there are supply-side faults).Indeed this is correct. Only on TT installations is it imperative to isolate both poles.On TN installations working with only single pole isolation seems to be widely accepted practice even if not strictly ideal.
Kind Regards, John