Traffic rules: Do I have to signal when turning is mandatory?. There are also intersections or junctions where turning is not a choice but an obligation.
Opinions differ as to whether you should use the indicator here too. When to flash: In these situations you also have to indicate the change of direction. The indicator does not have to be on while overtaking – the general rule for how long to start and stop is no. There is no rule for flashing when returning to the original lane.