Where only a single light is installed, it is somewhat pointless to install a ceiling rose or loop system.
Two additional connection terminals will be required - one for earth, one for neutral.
The two Earth wires are connected together in a terminal block.
The two Neutral wires are connected together in another terminal block.
The two Line wires connect to the switch - one in each terminal.
When the switch is on, both line wires are connected together through the switch, and the light will be on. With the switch off, the line wires are not connected, and the light will be off.
The earth and neutral wires remain connected all the time.
This arrangement is often used in locations such as garages and lofts, where only a single light is required.
It typically uses significantly less cable than other methods, and avoids the use of a separate junction box or difficulties in fitting an extra terminal into the light fitting.
It is also very easy to add additional lights, or to change the switch to a 2 or 3 gang to control other lights separately.
A three gang switch is used. The incoming supply line connects to one of the COM terminals. Short pieces of wire are used to link all of the COM terminals together.
All four neutrals connect together in a terminal block. All four earth wires connect to the earth block.
The line wires from each light connect to each L1 terminal, so that they can all be switched separately.
The diagrams above are for a plastic switch. If a metal switch is used, an extra piece of wire must be connected from the earth terminal block to the earth terminal on the back of the switch.