I agree that that's a good rule of thumb but as a hard rule it could create a dangerous situation. Each signal should only have one possible meaning
In short, you should always use a right indicator when overtaking a moving vehicle. Even if there are no other road users behind you, the driver of the vehicle you are about to overtake may benefit from your right signal prior to moving out. This is true whether you are on a single or dual carriageway. However, it is not always necessary to indicate left to move back to the left. If you’re on a dual carriageway, you may wish to evaluate the situation. If there is no one ahead or behind in lane 2 then you don’t need to indicate. However, with another vehicle following in lane 2, you would need to signal left before moving back across.
You must also consider whether your left indication could be confusing. As an example, if there is a slip road coming up then your signal may be saying that you intend to leave by that slip road. You may consider delaying moving back to the left until you are beyond the slip road.