Although the web version is hosted by NICE, the content is still compiled by the Joint Formulary Committee.
You will need:
Format:
Joint Formulary Committee (Year of publication) Title and number. Place of publication: BMJ Group and Pharmaceutical Press.
Example:
Joint Formulary Committee (2017) British National Formulary, 73. London: BMJ Group and Pharmaceutical Press.
In-text citation: (Joint Formulary Committee, 2017)