The way I see it is that , while traffic established on a main road has priority over traffic waiting to enter that road , a driver waiting to join that road can only give way to traffic he/she can see .
This is why it is extremely ill advised to overtake on the approach to junctions : since a waiting driver may judge that there is plenty of time and space to emerge safely ahead of a vehicle they can see coming , and cannot know there is someone behind , hidden from view , who is planning an overtake .
That overtake is clearly unsafe if both the overtaker and the vehicle waiting to emerge have no sight of each other and are going to come into conflict .
While both road users could have waited , the greater wrong , in my opinion , is in overtaking on the approach to / across a junction .
This is why it is extremely ill advised to overtake on the approach to junctions : since a waiting driver may judge that there is plenty of time and space to emerge safely ahead of a vehicle they can see coming , and cannot know there is someone behind , hidden from view , who is planning an overtake .
That overtake is clearly unsafe if both the overtaker and the vehicle waiting to emerge have no sight of each other and are going to come into conflict .
While both road users could have waited , the greater wrong , in my opinion , is in overtaking on the approach to / across a junction .