Most Box Junctions will be found at traffic lights.

They are there to allow the traffic to flow.

Which is the one reason why you should not enter them unless the direction of travel you are going in has a clear space for your vehicle to fit into past the box.

If you enter the box junction, and there is not space for you to clear it and the lights change, you will then be blocking the junction and other traffic will not be able to flow.

The only time where you can enter the Box Junction is when you are waiting to turn right and the road you are turning into is clear for you to do so but you are waiting for oncoming traffic to create a gap for you to move off, or you are waiting for a filter light to change to allow you to proceed, but the road on your right MUST still be clear before you enter the BOX.

Highway code rule 174 law TSRGD regs 10(1) & 29(2)

In the image shown, the vehicle turning right has moved into the box junction as they are turning right and the only thing stopping them from doing so is the oncoming traffic, once there is a gap for them to go they are in a better position to turn into the road on the right quickly. The option to go will come from either a gap in oncoming traffic or if a filter arrow in the direction of travel comes on. When the filter arrow comes on, the lights for oncoming traffic will have turned to red which then gives you priority.

The blue car could turn left as their exit lane is clear but they cannot pass the stop line if going ahead as there is a truck ahead and no clear space for the blue car to fit into without ending up stopping in the box. Once the truck moves far enough forward for the blue car to fit behind them without stopping in the box, the blue car can proceed.