As a fools rule i used to go by:
1. speeding, up to 20mph over the limit then fixed penalty.
2.Speeding between 20 - 30 mph over the limit i was going to court!
3.Speeding 30+mph over the limit i was also going to court but for a ban (ie for doing double the limit if in a 30 limit, or 100mph on a 70mph motorway).
Also another old wives tail is that the cameras are set at:
30 mph limit = 37mph
40 mph limit = 48mph
50 mph limit = 59mph
60 mph limit = 70mph
70 mph limit = 81mph
It equates to 10% of the actual limit, then 3 mph for speedo error then the next mph over your flashed! (30mph + 10% of 30 = 33 + 3 for error = 36, and camera is set to 37mph.
The guys who I used to work with would go by cameras like this all the time, I thought and still do think its a risky game for a few seconds of time. Plus ive bought a Road Angel now and its more acurate than any speedo so i just make sure im under 30 if im in a 30 camera zone.