AIUI, the requirements are that it cannot under-read at any speed, and for true speeds between 25mph and 70mph it cannot over-read by more than 10% +6.25mph.
Accordingly for any true speed outside of the 25mph to 70mph range, the requirements are simply that it must not under-read. If the result of this is that Doris drives at an indicated 8mph in a fraudulently misrepresented advisory 10mph "limit" and her true speed is 5mph, David Lammy will argue that that shows that the scheme is working.
N.B. If the general rule (for between 25 and 70) were extended to all speeds, Doris' indicated 8mph could lawfully represent a true speed of ~1.6mph, which would be argued to be a double win - although with no rule against the level of over-reading below 25mph, the speedo could lawfully display 80mph at that speed - which would result in Doris dying of a heart attack, as opposed to dying of old age before getting to the end of the road.