You are permitted to enter a box junction, stop and wait for oncoming traffic to pass, provided your exit road is clear.
It is against the law to enter the box junction if your exit is not clear as you will be blocking the junction for other road users.
Many box junctions around the country, especially in busy city areas have cameras fitted to take evidence of the incorrect use of box junctions.
If you are caught, you will receive a fixed penalty fine in the region of £135, although this depends where you are.
Local councils will often reduce the fine if you pay quickly or by the date provided.