GN1...
Where an accessory has an earthing terminal incorporated, the protective conductor should be connected at the earthing terminal. Accessories which have an earthing terminal incorporated, such as socket-outlets, normally have an earth strap connecting the earthing terminal to one or both of the fixing holes. Flush metal accessory boxes usually have at least one fixed lug (see Figure 7.8). The IET Wiring Regulations Policy Committee has agreed that such boxes may be earthed via the tight metal-to-metal contact of the fixing screw in the fixed lug. Consequently, unless both lugs are of the adjustable type, it is not necessary to connect the earthing terminal of the accessory to the earthing terminal in the associated flush metal accessory box by a separate protective conductor (i.e. an earthing tail).
However, as mentioned in a post above, it has to be a tight metal connection.