Primary position

Primary position is when you are riding in the traffic stream and drivers who wish to pass have to move to the next/oncoming lane.
Secondary is when you move to the left of the traffic stream letting a driver pass without them having to completely change lanes

This obviously depends on the lane widths involved. In a wide kerb lane of 4.5 metres, there is arguably no need to be in the primary position, as users of most motor vehicles can overtake with good clearance within the lane. Even for a 3 metre wide lane, a rider would have to be in a fairly extreme kind of secondary position for a driver to squeeze past within the lane. However, as the standard width for general traffic lanes tends to be 3.5m (in London, historic street widths obviously often make this impossible), which is in the range of 'critical widths' between 3m and 4.4/4.5m, where close overtaking can and does frequently take place within the lane, what you say is generally one of the most important considerations. (Also, close overtaking often involves only a slight movement outside of the lane, perhaps squeezing past oncoming traffic which in turn might move over slightly.)

Anyway, lane widths are to some extent a red herring, as while drivers tend to be very much guided by them (given that they fit the width of their vehicles; if traffic lanes were 10m wide, or roughly four times the width of their vehicles, drivers would be in a similar position to cyclists, who can move dynamically within lanes and tend to be guided by them much less), cyclists need not be, and they are like kerbs as fixed markers that aren't actually of the greatest importance in determining where one should be. I just mention lanes because there are certain desirable lane widths.