Pretending you don't know what speed you were doing is asking for trouble, they've then got every excuse to do you for driving without due care and attention!
A few years ago a policeman mate of mine was on his way back from a drugs bust one night about 3am, just coming on to the (empty) motorway and a Porsche went screaming past so naturally he put his foot down and pulled it over a few miles later. Asked the driver if he knew what speed he was doing and got "well I was showing 125 but if you want to tell me different I'll believe you". When asked why, the driver said "well…(long pause)…look at my car!!"
My mate told him that it was indeed a very nice car and if he drove it sensibly until they were out of sight then he could keep it… 😉
I got pulled over a few years ago on my bike for jumping 2 sets of red lights in quick succession on Elephant & Castle roundabout, the policeman let me go once I'd apologised profusely and swore that I'd never jump another light again.