ACPO guidelines followed by by most, if not all police forces allow a discretion of 10%+2 over the limit. So at
30mph you'll be booked at35mph
40 46mph
50 57mph
And so on.
Factor in a speedo overstate by 1/2mph I reckon you would need to be showing 36/37 on your speedo before you get done.
Not that I would chance it officer! 😀