simply to aloow for the cars speedo, the cameras are set to ACPO rules of 10% plus 1 mph. asif you r driver a new car with digital speedo, its normally correct to 1 or 2 mph, but an older analogue version has a tolerance of about 10%. So they are normally set to reflect this, also to buy a speed camera kit is about £35k each, where as a houseing is circa £20k, so they only buy 30-40% of cameras to housings, so many wont have cameras in.
In my area (west Mids) our council is so flush with money that on roads with a camera they installed another 1 or 2 everywhere, anywhere they could make a few quid. I agree with hotspots and schools, parks etc. but on a normal road on a bend (cant see it till its too late) you get the drift, over that last 6 months ive seen approx 20 -30 installed, then they close our swimming baths as they dont have any money...aaarrgggh, labour council for you.
Any way hope this helps, as its is considered a way for the government to get cars of the road, if your banned you cant drive. you have to use public transport. As you got yourself banned not them!