I got a minor in my test (2004) for going too slowly, I was doing 25 in a 30, this was on a clear/dry day etc so 30 was easily doable for that road, hence it was rightly a minor...
The OP issue does annoy me though, I drive down the A46 to/from work and the bit above the motorway at least is mostly NSL single-carriageway with only 1 or 2 'tight' corners (still doable at 60, in my ancient mr2 at least) and it irritates me to follow people who fail to do more than 50 on the straights and then break for the 'corners' to 35-40 ish.
I can understand it kinda if you don't know the road, but some of these people I see several times a week, doing exactly the same, frankly it's easy to go down that road at the speed limit without coming close to pushing the limits...