OP was taking about passports on the way in to country (schengen had passport checks on the way out, but UK doesn't) However Heathrow T5 (and T2 I believe) hav

OP was taking about passports on the way in to country (schengen had passport checks on the way out, but UK doesn't) However Heathrow T5 (and T2 I believe) have face scanners on departure, for domestic travellers. Passengers on a flight from Heathrow to Glasgow don't need a passport or any other form of photo ID (it's a domestic flight), but they mingle with international transiting passengers who have not immigrated into the UK (say flying Delhi to Heathrow to New York) To ensure passengers can't bypass immigration by swapping boarding cards, they take photos of domestic passengers on entry to security (either post immigration for Int-domestic transit, or post check in for domestic-domeatic). This is then compared with the passenger actually boarding the flight at the gate, and it's mostly automated (with manual override).