Most street lighting in the UK is supplied from the power company mains that serve other customers as well, not any separate or dedicated network.
Older mains are usually copper conductors insulated with oiled paper, lead covered and armoured with steel wires. Newer mains are usually plastic insulted.
Near each street light, a small cable to the street light is jointed to the main. This small cable is either 2 core with a grounded steel wire armouring, or may be single core with a copper wire armouring that serves a the ground and the neutral.