It's some combo of the latter. It's been a few years, but the last time I went through Heathrow, my face was scanned at passport control (arriving from US) and

It's some combo of the latter. It's been a few years, but the last time I went through Heathrow, my face was scanned at passport control (arriving from US) and then again at boarding (connection to Glasgow). The two photos didn't match enough and I was asked to stand aside while the gate agent figured out how to proceed (pull out passport for review). [at least that's my recollection of what happened]