I think there is a flaw in the advice on primary position in this (excellent) publication. "Cyclecraft"
carefully defines primary and secondary positions to be relative to the traffic flow, not relative to the kerb or any other fixed part of the road.
This is important to understand, for example, when passing parked cars. In this case, if you are riding 1.5 metres away from the parked cars, you are in secondary position, as you are probably riding on or to the left of the nearside wheel track of the cars. If it is unsafe for cars to overtake you in this position, then you should try to get into primary position, which is the centre of the traffic flow that is also overtaking the parked cars. This makes primary position 2.5 to 3 metres away from the parked cars, and half way between the wheel tracks of the traffic flow.
I did a blog article with videos illustrating the importance of this
here.