I feel a lot of commenters are simply focusing on what signs can do or what sort of junction to build, rather than focus on the fact that on high speed primary country roads, bicycles shouldn't be sharing the road with the cars.
In Denmark, a lot of high speed roads (excluding motorways, of course) out in the country have separate dual-way bicycle lanes near it but not attached to it. (Example: https://i.imgur.com/dS6jqXS.jpg)
That way, the cyclists can cross the side roads on their own accord, where they are more visible and have their own junction with the side road. Additional, one can set up a traffic light that only turns red for the cars when a cyclist is crossing (i.e. activated by a button).
Furthermore, one might consider signage that warns drivers about cyclists in the junction: https://i.imgur.com/CX6SJdW.jpg
Also, a way to reduce speed of the motorists without putting in stop signs would be to add chicanes just before the junction, so they are forced to slow down.
Plus, as I've mentioned before (see https://news.ycombinator.com/item?id=15977162), I also think it is because UK drivers don't really have enough training with how to deal with bicycles and the fact that they are also participants on the road.
In Denmark, a lot of high speed roads (excluding motorways, of course) out in the country have separate dual-way bicycle lanes near it but not attached to it. (Example: https://i.imgur.com/dS6jqXS.jpg)
That way, the cyclists can cross the side roads on their own accord, where they are more visible and have their own junction with the side road. Additional, one can set up a traffic light that only turns red for the cars when a cyclist is crossing (i.e. activated by a button).
Furthermore, one might consider signage that warns drivers about cyclists in the junction: https://i.imgur.com/CX6SJdW.jpg
Also, a way to reduce speed of the motorists without putting in stop signs would be to add chicanes just before the junction, so they are forced to slow down.
Plus, as I've mentioned before (see https://news.ycombinator.com/item?id=15977162), I also think it is because UK drivers don't really have enough training with how to deal with bicycles and the fact that they are also participants on the road.