This says you must abide by the 40mph speed limit given until you pass a traffic sign indiciating the NSL is in force.
That's interesting as that is definitely not how the system actually works, because you won't necessarily pass a NSL sign depending on how you exit the limit.
There are basically two scenarios to doing so. One is that there is a limit on a specific stretch, which you proceed through at (say) 40mph then exit. On exiting you see the NSL "computer zero" on the displays.
The other is that the limit is removed while you are on the 40mph stretch because the hazard/congestion requiring it has gone. In this case the signs are all just blanked, the "zero" does not show.
Because of the latter, and because of the conventions, I can't see any attempted conviction succeeding, nor indeed being attempted in the first place (particularly given that, as I stated above, the actual displays are echoed on the photographs via optical fibre).
For what you say to hold true the signs would have to either display the "zero" at all times when not showing a lower limit, or be set up to show it when a limit is removed until the last car that entered when it showed a limit had left.