> If they arrived here legally won't they have a passport? Or national photo ID at the very least? If they are driving for Uber or deliveroo then they'll have a driver's license too? If they don't have either (or a UK birth certificate) then it's safe to say they're not a legit citizen?

Simple. Overstaying or/and expired passport will lead to that. Valid status is not a fixed binary state. It is better described as a function of personal id, rights docs and current time. Currently, the checks are more akin to updating a Boolean column on rare occasions. Digital id countries do checks more like function calls that you can perform easily and quickly

RTW already requires ID plus NI, but OK what about if we just said 'free passports' and then said passport or driving licence plus NI is needed to get a job here. If you have a foreign passport and no UK driving licence then yes you'll have to keep that up to date in order to work here, c'est la vie.