The Institute of Electrical Engineers has published regulations which aim to promote safety and reduce the risk of injury from electrical accidents. They require that any metal parts of a building which could become live should be earthed.
Metal pipes conduct electricity, and can be dangerous if they become live by making contact with a faulty electrical appliance in any part of the building. Turning on a tap fed by unearthed pipework can cause an electric shock, for example.
For this reason, there should always be an earthing cable connecting the water and gas pipework to the earth terminal in the main consumer unit (fuse box). Also, wherever a non-metal component such as a ceramic sink or plastic pipe interrupts the circuit, some additional wiring is needed.
This is called Equipotential Earth Bonding, and consists of a yellow and green sheathed cable with a metal clamp on each end. The wiring is clamped either side of the non-conductive component to complete the route to earth, and is also used to join or “cross bond” pipes together.
Whilst your plumber will be able to remove and replace bonding clamps during maintenance etc, it is your responsibility to ensure a qualified electrician checks the installation is safe.