After having looked again I can see why OH takes the view that it is the General Service Corps which wore the Royal Arms as its badge, however my reference book says they were not formed until 1942,
Having looked again given that this soldier is in ww1 uniform I came across this reference to Royal Reserve Regiments,
In 1900 The Garrisons were emptied of first line troops for service in South Africa and they needed to be replaced by other troops so the Royal Reserve Regiments were raised, So without quoting chapter and verse and aware of the copyright etiquette, the Royal coat of arms was used for the Home Counties Northern, Southern and Eastern Regiments, So I will suggest that this soldier is from one of these units, precisely which one is not possible to say, I will say that I was not correct in my view it was the Manchester Regiment though it is easy to see the similarities in both badges and why OH came to the conclusion it was the General Service Corps and I to mine that it was the Manchesters.
James