How NOT to start a speed limit.

I will admit that what constitutes a speed limit starting at an arbitrary or non-uniform location is going to be seen as subjective.
However, I think there won't be many people who see the following images who won't admit that they can't at least see what I'm getting at.

I think I'll start off by providing a couple of examples for clarity of where I think a speed limit starts exactly where it should, and an example of a speed limit that changes in a seemingly arbitrary location. 

This one is on the approach to the Chewton Mendip on the A39 in Somerset.

 Gasp at the graciousness of all the nothing...

This starts in an arbitrary location, almost 300 yards from the village, there's no discernable change to the road's character here whatsoever.
That being said, due to visibility, this would be an appropriate location to place a sign warning drivers to start slowing down for an impending village and an upcoming speed limit reduction.

However, since there's no grace distance for speed limit signs, drivers should have already slowed down to the posted speed by the time they reach them. Unless you anticipate drivers exceeding the speed limit on the approach to the village, there is no need to set the signs back. If drivers aren't slowing down before reaching the signs, it may be a learned behaviour resulting from councils setting the speed limits too far back.

The speed limit also decreases to 40 another 300 yards further back for no apparent reason, which, again, makes even less sense for traffic heading North to adhere to. In some cases, where there is a small amount of isolated development on the approach to the village, there are buffer zones that consist of a short section with a 40mph limit. These make sense and, more crucially, they're logical in both directions.
If you have speed limits that make little sense for people to adhere to, how can you expect them to pay attention to limits where they do make more sense? This is likely to lead to the response, 'this limit is nonsense, I'm going to cheerfully ignore it'—precisely what you don't want to happen with speed limits.

What's more insidious is that where the speed limit should change there's no drop in speed limit in-line with a clear change to the road's character.
Drivers may be more likely to ignore or not notice a speed limit that seems arbitrary, and consequently, continue driving too fast upon reaching the village. Additionally, traffic leaving the village is instructed to maintain a 30 mph limit for no apparent reason, it's only because you're not allowed asymmetric speed limits on single carriageway roads.

Compare that to this, this is a good example of a speed limit change. It's in Wales and on my many travels around the UK I immediately saw it as a good example of how to start a speed limit and stopped to take a photo.

This, is how it should be done

Six Bells in Wales..

This speed limit makes perfect sense; it changes precisely in line with a clear and obvious change in the road's character. It also applies logically in both directions, visible along with the village from a considerable distance. This effectively communicates the message, 'you do not want to overshoot this.' I haven't got much more to say; this image is quite self-explanatory. To me, this is excellent

Does this make any difference?
I can understand the intuition behind doing this, people in a village may demand that the speed limit gets set back and a council may bow to the pressure.
I've not been able to find a single study that backs up this practice, and I have seen data that shows that it may indeed help exacerbate speeding through villages. 
When the village of Wraxall in North Somerset moved the speed limit start closer to the village in 2017, this had the effect of reducing speeds in the village and the drivers going speeds over 35mph went down by 95%!
What's disheartening is that many locals make an assumption that moving the speed limit to a more intuitive position would increase speeds through their village. Attempts to explain why this is not the case may be countered with Bulverism.
It can be difficult to convince them otherwise, so suggestions of moving the speed limit closer to the village can be met with the expectation that it might cause "carnage".

Unfortunately, the current trend seems to be setting the limit back even further or even completely removing sections of raised speed limits between villages. I believe this can contribute to making the roads in these villages more dangerous.