If you looked down at the speedometer instantly you could just be lucky with the tolerance they factor in because your speedometer is not calibrated to be exact.
I always thought this was something like 10% plus 2mph but I dont know where I got that from.
depends when they got you because i got caught by a van with the back doors open with a camera at the side of the road. when i got the letter it said i was caught at 45mph but i looked my speedo just before i saw them and i had got up to 50mph.