Here is the standard World War I sticker I normally start from:
{{The Great War |branch=British Army |startdate= |units=Army Service Corps |enddate=26 October 1916|rank=Driver |unit= Army Service Corps}}
If you have the "|branch=British Army" bit in there then it will add the flag automatically, you don't need to specify which image to use.
The "|unit= Army Service Corps" bit at the end will automatically put it in the correct category, as long as the unit is entered in exactly the same way as on the category.