there still is its your speed plus 10% speedo inaccuracy plus 2 MPH
http://www.safespeed.org.uk/speedlimits.html
see paragraph starting
Motorway speed limits
but these limits were set back in 1965, when the average car was a ford Anglia with a top speed of 85 mph on a good day, with drum brakes and a stopping distance of over 260 feet, most poorly maintained cars of todays age can half that,and things like the peugeot 106 with abs can cut this buy nearly a third again.
well said pidge its not the cars we need to worry about,limiting cars will only lead to people running older less economical cars that will do the high speeds,pointless