Although similar in appearance to modern cables, these have conductors primarily comprised of aluminium.
It is unusual to find cable of this type, as most of it should have been replaced by now.
It's only advantage was reduced cost during a brief period of very high copper prices.
The example here has been scratched with a sharp knife, revealing the silver coloured metal underneath (black insulated core).
The cable is substantially lighter than the copper equivalent and much softer to cut and bend.
This cable is no longer listed in BS7671 and information about it's current carrying capacity is difficult to obtain.