Originally Posted by
corporate-wage-slave
In both cases they should not be allowed to continue their journey without either the clearance of the Chief Immigration Officer or delegate, or can completely prove they really did enter from the UK and the technology failed on them. The last point (technical failure) can be delegated to supervisory HAL staff, the first point cannot be delegated beyond the Border Force.
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.