Dual carriageways. When crossing or turning right, first assess whether the central reservation is deep enough to protect the full length of your vehicle.
if it is, then you should treat each half of the carriageway as a separate road. Wait in the central reservation until there is a safe gap in the traffic on the second half of the road
if the central reservation is too shallow for the length of your vehicle, wait until you can cross both carriageways in one go
174. Road junctions - General
Box junctions. These have criss-cross yellow lines painted on the road (see'Road markings'). You MUST NOT enter the box until your exit road or lane is clear. However, you may enter the box and wait when you want to turn right, and are only stopped from doing so by oncoming traffic, or by other vehicles waiting to turn right. At signalled roundabouts you MUST NOT enter the box unless you can cross over it completely without stopping.
175. Road junctions - Junctions controlled by traffic lights
You MUST stop behind the white'Stop'line across your side of the road unless the light is green. If the amber light appears you may go on only if you have already crossed the stop line or are so close to it that to stop might cause a collision.