The central reservation can provide a space to wait while executing a right turn. (It can also be used as a mid-way point when going straight on across a dual carriageway.)
Before using the central reservation to wait, you must be sure it is large enough to safely accommodate your entire vehicle, including anything you are towing such as a trailer or caravan. If the central reservation is too small, do not enter it until you can safely complete the entire manoeuvre without stopping.
The exact position you take will be dependent on a number of factors including the size and shape of the central reservation, other road users, and any road markings that may be present. The layout of these junctions can vary considerably, so it is important to assess the situation carefully and be aware of what others are doing.
As for any right turn, a right signal should be given as early as possible without it being misleading.
When waiting in the central reservation for an opportunity to enter the side road, be aware that a right signal at this point could be misleading to other road users, who might think the blue car is going to turn onto the dual carriageway instead of into the side road.
In the diagram, the blue car’s angled position helps to avoid the right signal being misleading, so it is left on.
As with positioning, it is important to realise that these junctions come in many shapes and sizes. The example given here is not intended to form a definitive guide. You must approach each junction with care and consideration, and decide on a course of action which is appropriate for the individual situation.