To work, you need to provide a National Insurance number, which is unique and tied to certain state benefits like pension. The idea is you work, pay "national insurance" contributions and accrue "contributing years" to get a state pension later.

The wrinkle is that it doesn't seem to be tied well to identity. Someone working illegally can provide an NI number that's legit but not theirs. Their work accrues to someone else's NI record, but the person getting the extra years probably never notices and the person working under their NI number doesn't care because they aren't entitled to a state pension anyway, they just want to work now.

OK, this makes sense to me. Clearly I lacked some imagination on this whole front