This really isn't my area as it's rarely used in domestics.
However, I've found from one table on the net (
https://www.dungannonelectrical.co.uk/dun1-shop/pdf/cable/SWA.pdf) that 2 core 16mm will have a 42mm CSA of steel.
I am then not sure if you divide by 8.8 or 2.75.
AJJewsbury explains it below: (IET Forums - SWA Size of steel armour for CPC - https://www.------.org/forums/forum/messageview.cfm?catid=205&threadid=47544)
"If the armour is only being used as a c.p.c. (and not as PME bonding conductor) then you're not really interested in its actual resistance (either absolute or copper equivalence) - just its ability to survive an earth fault current for the required time - i.e. it's "k" value (tables 54.2 and following). E.g. if the cable runs at max 70 degrees in normal service with a maximum ambient of 30 degrees, then k for copper is 143 and steel 52 - making copper just 2.75 times "better" than steel for the same c.s.a.
So if the cable had 76mm2 of steel that would be the equivalent of 27.6mm2 of copper - easily satisfying table 54.7. "