From Genuki:
'Stillborn children were not registered prior to 1927.
Stillbirth registration was introduced on 1 July 1927 to help protect infant life, provide a valuable source of statistical information and to give parents the opportunity to have their child officially acknowledged. A stillborn child is a child born after the 24th week of pregnancy who did not breathe or show any other signs of life. When a child is stillborn the midwife or doctor will issue a medical certificate of stillbirth which will be used to register the stillbirth.
When stillbirth registration was introduced the the age limit was the end of the 28th week of pregnancy, not the 24th (as it is now). This is a relatively recent change following the greatly increased survival rates of premature babies.
Current GRO policy on obtaining stillbirth certificates: "Due to the sensitive nature of stillbirth registrations, the procedure for ordering a certificate of the entry differs from other types of certificates. We will only send out the application form after we have been contacted by phone or in writing by the mother or father (if he is named on the certificate). In cases where the parents are deceased, a brother or sister can apply if they can provide their parents' dates of death."
I would phone the regsiter office where the stillbirth registration would have been made and ask for their help in checking their local index,