The current set of Construction and Use regulations (the regulations that manufacturers are required to build vehicles to) state that a speedo must be accurate to +-10% at 70 MPH, by implication (not statute) the +-10% cascades all the way down (as a 10% errored device at 70 is hardly likely to be spot on accurate at 30).
The +10% requirement is the reason you you cannot legally be nicked for 33 in a 30 zone, the +2MPH (on top of the +10%) was the police's own addition when hand held radar guns were 'dodgy' (AKA unreliable) near the limit of the zone and +2mph guarantees zone +10%+1mph.
In short if your vehicle has the manufacturer's speedo fitted, plod cannot nick you for being at +10% of the zone limit - because your vehicle complies with the C&U regs, so a conviction is therefore unsound legally.
Sadly most plod do not know the proper C&U regs (and even less so as they rise up the management tree), so for any plod to say we'll nick at +1 MPH of the zone will only give rise to lots of defendant solicitor costs being awarded against plod - that's why this crap all went away last time one of 'the managemnet' uttered the same drivel.
The ONLY way to nick at zone +1mph is to change the C&U regs and then scrap or waiver all the vehicles built to the previous regs.