Mainly to combat the requests from other players to introduce other British Dominions (i.e Canada, Australia etc.) to the game, it would be easier just to rename the British faction to an appropriate name.
Another factor for this change that could be argued is that generally most Commonwealth forces fighting the war used mostly identical equipment to their United Kingdom overlords.
Battlefield One and V got this (surprisingly) right by calling that faction "Commonwealth" as it acknowledged their efforts during both wars.