https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-hong-kong-british-national-overseas
HK 15.1. The applicant must be:
(a) the child of a parent who has, or is at the same time being granted, entry clearance or permission as either:
(i) a BN(O) Status Holder; or
(ii) the partner of a BN(O) Status Holder; or
(b) the grandchild of a grandparent who has, or is at the same time being granted, entry clearance or permission as either:
(i) BN(O) Status Holder, or
(ii) the partner of a BN(O) Status Holder.