The Highway Code is a valuable resource which you should check often.

Signals to other road users.

Be very careful with your signals.

It not only matters that you indicate your intentions to other road users but also when and, sometimes, for how long.

Direction indicator signals
I intend to move out to the right or turn right I intend to move in to the left or turn left or stop on the left
Brake light signals
I am applying the brakes
Reversing light signals
I intend to reverse
The above signals should not be used except for the purposes described.
Arm signals
For use when direction indicator signals are not used, or when necessary to reinforce direction indicator signals and stop lights. Also for use by pedal cyclists and those in charge of horses.
I intend to move in to the left or turn left I intend to move out to the right or turn right I intend to slow down or stop