the key differentiation here is where are you a tax resident for when you earn or get that income.
Person A: UK Passport Holder, but not residing in UK (i.e no tax residency in UK) - Any income earned outside of the UK is tax exempt from UK taxation ... so anything you earned in HK "during the period you were not a resident in UK" will not be taxed by UK.
Person B: UK Passport Holder, with a tax residency in UK (see HMRC for specific rules that establishes tax residency in UK) - World Wide income needs to be declared to HMRC and applicable tax to be paid. Declaration is voluntary, but if you do get audited or "found out" then you start shitting bricks depending on the amount you did not disclose.
Person C: Not a UK Passport Holder, but a "resident in UK" - Same rules as Person B above.