Usually your allowed 10% over the speed limit but I'm not sure if that still applies today - I'm constantly weary of mobile police units (they seem to hide in dustbins and pop out when you least expect it ) where I live anyway lol
ACPO guidelines for prosecution is 10% + 2mph as said... the guideline suggests not to prosecute under that, but they probably still could if they wanted to.
I hear they treat you really nice in prison. All the inmates smile big at you and generally go out of their way to make you feel welcome. Don't worry about the soap bit, they use liquid as it's easier to get into crevices with that.