For pedestrians, yes that type of road is fairly dangerous. But for cars it's not too bad, drivers know they need to pay attention and nobody is going at high speeds. It's also easier than you might think to see oncoming cars, especially when it's dark and their headlights are on.
Fortunately/unfortunately that type of road is becoming slightly less prevalent, as efficient modern farming means hedges are maintained less than they were historically and they either get chopped down or neglected and end up growing into larger trees that are much easier to see through.
TBH it doesn't seem too safe for cars either. You couldn't possibly see deer or other animals until they had already jumped into the road. Any normal hazard like a car stopped for a flat or lumber that fell off a truck is more hazardous in this situation.
Honestly, these are not common occurrences in England. Cars that are stopped like this are required to post a warning triangle ahead, and lumber just doesn't fall off trucks (lumber isn't very common anyway as the country is mostly devoid of forests).
While I agree that they're bad for sightlines, they're also traditional and may be on private land, so getting rid of them from minor roads would be a hugely unpopular undertaking.
It's interesting to discover a seemingly avoidable hazard to which the British are sentimentally attached. Perhaps it's not quite comparable to our attachment to firearms.
