Well even though members of the Met have been diciplined for using it, seemly the far knowledgable sages on nSC seem to think it is not, so I would go with the NSC rather than the Met!
Anyway nitty gritty was a term used to describe the state of the slaves in the bowels of the slave ships...
The sister in law of a guy I work with is a high ranking officer in the met and he told me that three of her officers were disciplined and had to go on a race awareness course for using the term , perfectly innocently, during a meeting.