SWA (Steel Wire Armour) cable. This has steel wires running along the outside of the current-carrying cores. These steel wires are earthed at at least one end of the cable. So if someone shoves a spade through it, the spade will conduct very high current from the live cores to the earthed armour and trip the circuit breaker almost immediately, ensuring nobody gets hurt. It is also far better protected against impact. The ends of the cable are terminated in special glands, in order to earth the steel sheath properly.
Also see:
SWA Earthing
and:
Electrics:SWA_gland