when i passed my driving test back in the day i had 2 minor faults and one of those was driving too slow for the conditions, and too be clear i wasnt doing 30 on a national speedlimit it would have been around 50. it was a while back so i may be wrong but 2 minors (or was it 3?) in the same category would have been a fail
So whilst you may be correct from a gonna be prosecuted kind of way, driving below the speed limit when there is no reason to is definitely considered poor or inconsiderate driving (otherwise why give a minor fault for it)
For what it’s worth, sister’s ex husband used to be a driving instructor. Among some of the insights he shared was that he had to remind learners that driving at 20 in a clear 30 mph could result in a failed driving test.