With respect, I think this is semantics. The gov.uk page is there to be as understandable as possible, rather than being technically legally perfect. As we will see, the defendant does need to give a written reply with their declaration indicating whether they are saying they are guilty or not.
Yes, or in normal words, a response to the SJPN of the sort that the defendant would have provided if they had been aware of it.
Nothing in that section suggests the written notification is in any way "preliminary". It is the only response to the SJPN. There is no provision in law for the defendant to later give a second/subsequent "full" response.
Or, in normal words, (1) saying you are guilty and you do not wish to appear in court, (2) saying you are not guilty, or (3) saying you are guilty and you wish to appear in court. In normal words, a plea.
The usual effect of the response (1) above is that the SJPN magistrate will proceed to convict and issue a fine. For (2) is that a trial will be listed and the defendant will be summoned to attend. For (3) is that a sentencing hearing will be listed.
In summary, when following a section 16E statutory declaration process, there is no "interim" time between serving the statutory declaration on the court when the case is somehow in "limbo" before the defendant must commit to saying whether they accept they're guilty or not. These two things are to happen at the same time. Whether it is technically a "plea" or not is nothing more than semantics.
You seem to be taking the position that the 'written notification' required by Section 16E(3)(d) amounts to a plea and that the accused's response can only be between the choices of 'guilty' or 'not guilty'. If the response required to an SJPN was intended to be confined to that binary choice, then you should ask yourself why the legislation was not so worded as to require a written 'plea' rather than 'notification'. The very fact that Section 16A does not call upon the accused to submit a written 'plea' indicates to me that the accused is being called upon to submit something else by way of response.
Furthermore, regardless of whether the accused has initially indicated by his 'written notification' a readiness to accept trial of his case under Section 16A, Section 16B(2) effectively confers upon him an absolute right at any time prior to trial to halt that way of dealing with the matter by giving notice that he does not desire to be tried in accordance with Section 16A. This is indeed a provision in law for a defendant to give a second response to the SJPN, and to this extent at least the written notification prescribed by Section 16A(1)(d) does not commit the accused irrevocably to a Section 16A trial in his absence in which he is taken to have entered a guilty plea by virtue of Section 16A(8). Instead, the court is required following receipt of a Section 16b(2) notification to issue a summons requiring the accused to appear in court for trial of the written charge. It appears that at that juncture Section 9(1) is engaged and requires the court, if the accused is present, to " state to him the substance of the information and ask him whether he pleads guilty or not guilty." That is the point at which I regard a plea as being taken, and I can see no basis on which, if the accused pleads 'not guilty' at that stage, the court could properly treat his previous readiness to accept a Section 16A trial as an irrevocable guilty plea, as the view you are taking seems to suggest.
From the practical viewpoint too it may be thoroughly unjust to treat an accused's 'written notification' in response to the SJPN that is filed with a statutory declaration under Section 16E as constituting his plea to the charge. At the time of making the declaration the accused will frequently have little knowledge of the nature of the case against him other than the fact that he has been convicted of a particular offence without prior notice of the proceedings leading to that conviction. So it may be entirely appropriate for him say that he is not yet in a position to make an informed decision as to the nature of his plea when filing his SD and written notification.
A notification to the court that you do not wish to be tried in accordance with Section 16A is just that; it amounts to an assertion that, irrespective of whether you ultimately decide to plead guilty to the charge you face, you do not wish the matter to be dealt with in your absence under the Section 16A procedure but instead desire the case to be dealt with at a hearing at which you can appear. That is entirely different from "saying you are guilty and you wish to appear in court. In normal words, a plea."
You call the difference between 'plea' and 'notification' semantics, I call it one involving words that have substantively different meanings.