David Hansen wrote:

The electrical installations in many buildings rely on the cable
armour for earthing.

Indeed they do, but the point to be made is that using the armour as
earth (CPC) will always require design calculations to verify earth
fault loop impedance (for disconnection time compliance) and that the
armour will not overheat before a fault clears (adiabatic compliance).

OTOH if a copper cable core is used as the CPC you have a CPC which is
equal in size to the live circuit conductors and calculations aren't
required (provided the feeding fuse/MCB rating doesn't exceed the
current rating of the cable). The maximum circuit length (to meet both
voltage drop and disconnection time requirements can simply be looked up
in Table 7.1 of the OSG.

--
Andy