It CAN vary depending on which county you get's nicked in !!
Also, a plod will take in to account how you were driving (i.e. pratt = more chance of being nicked - sensible, decent weather, road etc, just too fast = possible fix penalty)
My ex boss got away with a £60 & 3 point for doing 130 on M5, where as a guy where I work just got court summons for doing 70 (in a van on a dual carriageway) which was only 10mph over (for a van)
That's a point, does a series or defender with no windows count as a 'van' in law, so would have to stick to 60 on a dual carriageway ????
(assuming it got to 60 in the fist place

)