There's one in Rockingham (I think - Home of the Speedway... or on thar road anyway) thats where you got either from a 50 or 60 down to a 30, immediately down a pretty steep hill and round a bend and WALLOP a Truvelo (the one that looks at the front of the car)! I was told to watch out for it and it still took me by surprise. not sure on the range of those, but i guess by the time you see it, it's seen you...
I've long since lost my 6 points (SP50, 99.51mph on the M40, 8am Sunday morning...). I know the reason - I had a faulty speedo (it was accurate to 74mph, then didn't go any further...) but never try and use that as an excuse as you stand a chance of getting nabbed for not having an accurate speed indicator in the vehicle! My Dad was a Police Mechanic some years ago and the amount of cars which ended up on their rolling road for checking was amazing...Most of them with a nice shiny set of new Alloy wheels, or had just had an engine & box swap...