[mnahkies]

The nuance is that you can have a NI number, then have your visa lapse for whatever reason - you still have the NI number. Hence the requirement to prove your right to work through another means.

Previously you could use proof of British nationality or a physical biometric residence card - but they've been replaced by the digital share code system (which tbh hasn't been too bad)

[IanCal]

No, those are still the ways of proving you have the right to work, it’s only if you’re not a national that you need the share code.

[mnahkies]

Sorry I worded that poorly - I was trying to make the point that citizens prove their right to work using passport/birth certificate, and until recently visa holders used a physical BRP, and now a digital system (which oddly enough uses your expired/redundant BRP number as a username)