I got a nice scribble on my license and a £60 fine for doing 35 in a 30 a few years ago, so I'd say the 10%+2 or what ever is purely discretionary. I was flashed by a Gatso, so received no discretion whatsoever.
I was given the opportunity to do one of the silly courses then, but it would have cost me more than the fine in fees and travel, plus a days holiday from work. Utterly pointless waste of time.