CAP413 is renowned as being a frustratingly inaccurate document, despite its constant revisions. Many chapters have techniques in conflict with MATS phraseology.
It even gives an example of an acknowledgement from an aircraft to ATC being read as "G-ABCD." ie callsign only! What an ancient and nonsensical suggestion.
It would seem the person tasked with revising CAP413 is a Monday afternoon part time retiree or certainly someone who doesn't attach much importance to closing the loop between CHIRP and MATS etc.
PS My take on the subject matter is that
pressure settings with a value of 1000 or less should be suffixed with the word millibars. This means one uses "one tousand millibars" to avoid any confusion with an altitude as the suffix has been included to remove the potential ambiguity. Can't remember if I was trained this or read it somewhere but if someone can quote different from an official source I am open to changing my method.