The threshold is generally accepted as 10% over, so 77mph on a motorway. However, I only know that from hearsay, so take it with a pinch of salt.
Again, no credible source.
Dad, if you read this I didn't mean it, you are very credible.
Made up: I thought the 10% came from the rules the speedometer is calibrated against which are +10% -0% i.e. Your speed will be 30mph but you could be doing 27. You will never be doing 31.
I guess the plus number is whatever the council have chosen to enforce. Another person from the country thinks it's 10%+2
edit: Here you go, it's a more nuanced than above. Europe's law. Read Page 8 - http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/...
Displayed (V1 ) and the true speed (V2)
0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h
It used to be generally accepted that you were safe doing 80 mph but with the proliferation of average speed cameras people are a bit more conservative.
10 years ago it wasn't unusual to see people doing 90 - 100 on a motorway - now it's rare.
Nope. The limit is really 70, however to take into account possible inaccuracy in your speedometer or the traffic camera, there's some leeway in place. If you start assuming the limit is 79 then if nothing else, you no longer have the benefit of that leeway. How sure are you that both your speedometer and the camera are 100% accurate?