Rule 174

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.

Law TSRGD regs 10(1) & 29(2)

Rule 174: Enter a box junction only if your exit road is clear