https://commonslibrary.parliament.uk/research-briefings/cbp-8939/
Who has British National (Overseas) status?
The British National (Overseas) – ‘BN(O)’ – status was created as part of the arrangements relating to the handover of sovereignty of Hong Kong on 1 July 1997. It caters for people who, prior to the handover, had British Dependent Territories Citizenship (BDTC) through a connection with Hong Kong.
Put briefly, Hong Kong BDTCs lost that status on 1 July 1997. However, in the ten years leading up to the handover, they could apply for the newly created BN(O) status. It is no longer possible to apply for BN(O) status.
Although there was an entitlement to be registered, BN(O) status was not conferred automatically. Those eligible had to submit an application. Successful applicants were issued with a British passport displaying their BN(O) status. They were not issued with a stand-alone certificate of registration.
BN(O)s cannot pass on their British nationality status to future generations. Consequently, the status has been available to a fixed cohort of people which is gradually decreasing in size.
Slightly fewer than 360,000 people have a valid BN(O) passport, as at April 2020. The government estimates that there are around 2.9m BN(O)s currently living in Hong Kong. Hong Kong’s population is estimated to be around 7.5m.
Most people with BN(O) status are also likely to be Chinese nationals under Chinese nationality law.