A. Traffic should use the hard shoulder
B. The road bends to the left
C. Overtaking traffic should move back to the left
D. It's safe to overtake
These painted arrows indicate that you must move left if you are overtaking. It can be for a number of reasons. In the case of the image above, there's a hatched median strip marked on the road like this one on the approach to Rottingdean.
You can see in the image below, again there's an area of chevrons ahead separating traffic.
They may also be present if the road will become a dual carriageway, if there's a junction or roundabout ahead, if the road narrows or has a significant reduction in visibility, or if a lower speed limit is ahead.
In the hatched area above it is still permitted to enter it to overtake because the lines are broken. You would not be able to enter it if the lines are solid.