Interesting point Bert. I probably included that one last time and probably should have also had it in mind this time.
For practical purposes, guessing who was the driver, tossing a coin, or choosing the most convenient driver if unsure, seems to avoid a lot of hassle and likely reduce the eventual punishment.
However, subject to the wording of the s. 172 notice (and interpretations thereof), the respondent is effectively falsely making a statement to the effect that they "know" that the named driver was driving. And they are not "providing any information that is in their power to give and that might lead to the identification of the driver", so if they have not coincidentally named the actual driver, they have also committed the s. 172 offence and negated the possibility of any defence.
Some regulars are absolutely adamant that naming the most likely driver is absolutely fine, but that naming the most convenient potential driver is perverting the course of justice. In both cases, the person making the statement that he neither knows to be true or untrue. The difference appears to the the probability that it is correct.
Simon's response appears to mostly be using more words to restate the distinction I noted in the OP. Perverting requires a positive action, so sitting on your hands and being convicted cannot constitute perverting. The argument that pleading guilty - which is a positive action - cannot constitute perverting when you know you did not commit the offence because you would be convicted if you simply sat on your hands, albeit with a higher punishment, seems to be somewhat perverse. Effectively, if something would otherwise be perverting, but the result could be achieved without a positive action, that negates an offence of perverting which included a positive action.
As a hypothetical example, A is the RK and B was the driver. B intercepted A's NIP and responded as A naming "himself" (A) as the driver. A discovers this and for some reason is keen to avoid throwing B under the bus. A receives a summons (or SJPN). A is faced with the practical choice of pleading guilty to the speeding, pleading not guilty and sitting on his hands, or pleading not guilty and defending himself - which would throw B under the bus.
As A would be convicted of the speeding offence that he did not commit if he sits on his hands, the argument appears to be that he is entitled to plead guilty to an offence that he knows he did not commit in order to lessen the eventual punishment (and effectively to protect the very guilty).
I would suggest that the majority of the hive mind would say that the position above "cannot be right", and I would agree. However, legal principles apply to the facts, rather than the other way around. If A cannot plead guilty when framed, do the same principles apply when he wasn't framed?
I would strongly suggest that for the purposes of this discussion and general sanity, we assume that something is either known or not known - A knows that he was driving, knows that he was not driving, or is unsure which of the possible drivers, A, B or C, was driving.
I have also semi-deliberately avoided potentially complicated the issue by inviting discussion on what the course of justice actually is.