I am having some difficultly with world war one records on Findmypast. I looking into William Harris who was born on 3 February 1890 in Havant. He still living in the parental home in 1911 working as a skinner and then doesn't seem to appear in the 1921 census or 1939 Register. I discovered that there is the name William Harris on the Men of Havant war memorial in St. Faith's Church in Havant, but it's just a name, no other details.
I then found a record of a William Harris from Havant who died in Mesopotamia on 11 September 1916. That record has a service number 280762, but when I search for the service record to match that service number, I get two results but for completely different names.
I have also found the Commonwealth War Graves information for the William Harris who died in Mesopotamia but unfortunately there is no additional information on that records either.
It all seems possible, but it would be could to find something with an age, date of birth or next of kin that would confirm it as a match.