i dont have any idea of the offical answer, nor do i really care for it that much, but heres my tuppence worth...
to isolate a circuit requires the live to be disconnected and locked off, a NEUTRAL is also a LIVE
to do it properly would require a DP MCB,
to do it practically would be the SP MCB we all use, as the main switch means killing every circuit thus causing untold inconvenience,(even in a household, never mind offices/factories etc.)