The relevant police force does get a kickback (£35) from the cost of a Speed Awareness Courses.
http://www.telegraph.co.uk/news/uknews/law-and-order/11980318/Speed-awareness-courses-could-invalidate-insurance.html
It used to be the case that most speed limits were 70,60, and 30, and you could tell which it was by looking at the road. (And this is mostly true in some other countries).
Now, it's much more difficult - speed limits can change every mile or two, and are often not related to the layout of the road, so you have to constantly be looking for signs and keeping track of the current limit.
It's not surprising that if you go past dozens of changes in speed limit in a day that you might miss the occasional one. And the speed trap will often be just inside the limit, before you get a chance to see any repeaters.
Even careful drivers trying to stick to the limit can get caught out. Everybody makes an occasional mistake, particularly if there happens to be other hazards that are momentarily more important than looking for signs.
Driving too slowly is only unsafe when other drivers are impatient or unobservant. If everybody was driving slower, there would inevitably be fewer accidents.