What does the beginning of BC mean in passport number?

The beginning of BC in passport number means British citizen (BC), British citizen.

Other possibilities are:

1) British Overseas Territories Citizen, former British Dependent Territories Citizen (BDTC);

2) British Overseas Citizen (BOC) British Overseas Citizen;

3) British nationals (overseas) (BNO) British nationals (overseas), generally Hong Kong people only have BNO status. If British nationals (overseas) in Hong Kong become stateless after reunification, they can register BC status according to the British Nationality (Hong Kong) Act 1997;

4) British nationals;

5) British Protected Persons (BPP) are British protected persons.