strangley enough my satnav also records a lower speed than my 96 Laguna Green Machine, by approx 3-4 mph at around 30-40mph. As you get faster the difference between speedo and satnav increases, to the point where there is approx 10mph difference (i'm not saying how fast as you'll tell me off)
However - i get in nearly any Beemer and the sat nav is nigh on spot on with the speedo!!!
RE fines points - France and the UK will or already are arranging reciprocal arrangments for the recovery of fines and points - there is info on the DVLA website about it and a report was shown on one of the more informed driving websites!