All the gantries have cameras on (and working) I've heard, and they are still working when the speed limit is at national speed limit. The one issue I have is that I quite often don't see any national speed limit sign displayed to denote the end of the variation. Technically if you have seen a gantry with a speed limit displayed and then the next gantry without one, the limit still applies and this should be the case until the national limit is displayed. However,
gov.uk has a page on how to use a smart motorway and this section is interesting ...
"A speed limit displayed inside a red circle is legally enforceable. If you go over the speed limit, you’re breaking the law and could receive a fine. We’ll let you know when it no longer applies by setting the national speed limit sign.
If no speed limits are displayed then the national speed limit applies. When speed limits are displayed, they remain in place until notified that they have ended, or a different speed limit is displayed."
If, as you say, only 2 gantries were lit over a range of 20 miles when there were other gantries not displaying, and there were no other obvious reasons for the reduced speed (roadworks, heavy traffic, weather conditions, messages about obstructions etc) then it would be interesting to see if the argument that you were concerned that you were now driving too slow for the conditions would stand up and believed in good faith that the national speed sign had not been displayed due to an error, akin to when temporary traffic lights fail and drivers have to break the law in order to keep the traffic flowing.
Probably won't work if you were caught by the gantry displaying one of the messages, though.