I'm amazed its taken this long for someone to trot out the 'its a limit not a target' line. There is a reason why the 'limit' is there. It is not safe to drive some 30/40 miles an hour under that 'limit'. If you aren't confident to drive at least somewhere close to the 'limit' you should NOT be...
About a year ago on the 272 I was stuck about 7 cars behind a 2mph Jazz nightmare. Right behind the offending low tax high mpg blue rinse mobile was a police car. Doing nothing about it at all. Really really winds me up!
Its starting to drive me to drink this. Cars (normally a Honda Jazz/Nissan Micra, although there are others!) that seem to drive at a steady 10-15 miles under the speed limit constantly. In my mind if these drivers don;t have the confidence to drive at the speed limit or at least something...