Terms googled: Wording on the First World War beverment notice, First World War beverment notice (in google images)
For a British soldier, who is a Lieutenant, to his mother.
I am pretty sure that it would be something like:
We are sorry to inform you that [name of soldier] of the [regiement] died on [date]. The military offers its condolences at this sad time.
But can anyone give me something more specific?
Ta very much.