OK, I get it now, it isn't about whether they are in the UK legally or not but about demonstrating that they were already in the UK before boarding the flight.
Regarding the point about "technical failure": at the time biometrics failed on me at LGW-JER, I wasn't a British citizen yet, but I wasn't asked about my nationality - which fits in with the fact that they only sought to establish whether I was in the UK before going airside.
However, I was not able to completely prove I really did enter from the UK.
I offered my driving licence plus a bus ticket from the day before, which I could easily have received from a nefarious BP swapper. I didn't bring my passport.
Presumably they just checked the cctv for security to see that you entered earlier and therefore came from landside and must have already been in the UK.