speeding is speeding. 1mph over is speeding and is against the law.
the charging guidelines do allow some discretion, and these vary by area. my local ones are here
Questions | West Yorkshire Police . this seems to be 10% + 2mph in West Yorkshire.
whatever the speedo says is largely irrelevant but the manufacturers need a standard to work to. I am certain that if they just had a guess at it there would be endless complaints from users receiving speeding tickets.
I dont know, but I would expect that on the motorway in dry conditions, the police are not that interested in the traffic at 80mph, they will look for the car that is travelling significantly faster than the rest, or weaving between lanes.