Did real English soldiers really wear plastic dog tags?
According to Wikipedia:
First World War
The British Army and their Imperial forces in Canada, Australia and New Zealand issued identification tags from the beginning of the First World War. The tags were made of fibre, one in red and one in green and suspended around the neck by butcher's twine. The same pattern was worn into the Second World War and the Korean War by Commonwealth forces.
According to Physicsnetbase:
Fiber reinforced composite materials encompass a wide range of material classes from reinforced glasses, plastics, and rubbers.
So, Emily's grandfather very well could have been issued plastic dog tags during WWI. It seems like AG did more research than one would think.