According to american logic, the british three-letter appreviation of September is "sept":
datetime('05 Nov 2021','InputFormat','dd MMM yyyy','Locale','en_GB')
datetime('05 Sept 2021','InputFormat','dd MMM yyyy','Locale','en_GB')
PS: to be fair, the DATETIME documentation does not state that abbreviations must be three letters (although I suspect that many english speakers would assume that it does), it actually states "MMM Month, abbreviated name", thus allowing any length abbreviation.
However the documentation does not list those abbreviations anywhere, nor the full months names or anything else, but it does link to the Unicode documentation, where if one spends enough time digging, one might find a reference to how dates are abbreviated in every supported language (I cound not find it).
