His service records will still be held by the Ministry of Defence (all post-1920-ish records are still with the MoD)
It will take some months to get the records but they will give you the full story, and are the only people who an do that. But please note that
1. You need to supply a death certificate;
2. Things tend to be easier if you are the next-of-kin or have the permission of the next-of-kin because some stuff is withheld until 25y after the serviceman's death unless
you are, or have the permission of, the next-of-kin.
Your first task then, is to get that death certificate - without it, the MoD don't know whether the soldier has died and they won't assume he's dead until X years have passed since his birth - I don't remember what X is and couldn't see it on that link, but it is several years more
than 100 years.