Any light, lamp, bulb holder or fitting with a conductive or metal exterior surface must always be earthed to ensure against risk of electrocution or serious injury.

How do you know if a lighting circuit is earthed?

You can check at the consumer unit if you think the FEED cable has a missing earth. With the power off you should note the cable going to the mcb and trace the earth wires back to the earth terminals. Remove both earth wires from that circuit and check there is continuity from one to the other.

Why is earthing not required for lighting circuit?

If there is some unwanted power signal (noise) in the wire it can damage the device. To reduce this effect earth is necessary. Lighting circuit carries low power (current). So, we ignore the earth terminal.