Found this. No doubt some of you knew this fellow.

History of the .303 British Calibre Service Ammunition Round

Short service history with BP before Cordite comes along, but Nitrocellulose is right there too. Given reason per the history was Cordite being superior in warmer climates (Empire business)

Real reason would look to be more about having a home grown solution, regardless of other considerations.

History of Cordite Production