There's also a legal requirement to report a birth within 36 hours. In practice that will usually be done by the midwifery service, but in this case that won't have happened obviously.
I don't know if there's any penalty, but IMO a failure to report a birth leaves a child vulnerable to abuse, as it's difficult to make arrangements for intervention to protect a child who is undocumented, and so in itself would be a social services red flag, all other considerations aside.
Birth notification service - NHS Digital