Almost all mobile speed traps are set up in 30's and 40's. With very few exceptions, a 30mph area is fully built up and, if not, is always adjacent to a bulit-up area. Approaching a 40 is always an indication of an urban area or the outer approach to one. If you are speeding in a 30 or a 40 area, you are, almost without exception, driving past people's homes with the inherent dangers of children, the elderly or pets or other vehicles performing slow, hazardous manouevres. That's why they are 30/40mph zones.
If the whole country was a blanket 30 on single carriageway roads, I would agree with the claims being made regarding revenue raising, but this is a safety issue. Please note that I think that many fixed camera sites are revenue raisers, though (if they are outside the 30 or 40 limits).
People mostly like being stopped by "real coppers" because there is a chance to blag their way out of the problem - not because they appreciate the "friendly word of advice" and the policman's discretion. In my yoof, I was often stopped by coppers (the good old days before cameras) and as soon as we had had our friendly chat and I had tucked maybe 5 miles of contrite look-at-me, goody-two-shoes driving under my belt I was off harry flatters again - most people are, let's be honest.
Driving is about being sensible but it is also about following rules. If you can't drive at thiry in a thirty then not only are you showing an inability to do something really rather simple to do, but you are showing a blatent contempt for the lives and the wellbeing of others.
And as for the "it is dangerous to brake" argument, well
why not disconnect the pedal and be a really safe driver.
Philip