Many are perplexed about the figure of "3 million Hongkongers" under Boris Johnson's BNO pledge. Here's a short thread about what is and is not covered in the magic number:

The number is made up of two parts: (a) Those who currently hold a valid BN(O) passport, and (b) those who used to have one but have ceased to renew it.

Category A: As of 24 February 2020, there are 349,881 holders of BN(O) passports, according to the Home Office.

Category B: Another 2.5 million people can renew their already expired BN(O) passports – and then follow Johnson's path to citizenship if they so prefer.

Now we come to another group of potential beneficiaries: Dependents. They also comprise of two groups: Spouses, and children under 18. According to Dominic Raab they can also move the UK, even if they are not BN(O) holders themselves. They are not part of the "3 million" figure.

Caveat: The UK government's 3 million figure is potentially inflated, as some of these BN(O) holding Hongkongers have already passed away, or obtained other passports such as Canadian or Australian. Such situations are not necessarily known to the British authorities.

One final thought: As Raab pointed out, many BN(O) holders, current or eligible, would decide to stay in HK, so it's too early to say there'll be a mass exodus. From a public policy POV, however, the UK might consider attracting these BN(O)s to different parts, not just London.