A few years ago I experienced something similar at Heathrow. According to Heathrow's website[1] it's because: airports that operate common departure lounge

A few years ago I experienced something similar at Heathrow. According to Heathrow's website[1] it's because: airports that operate common departure lounges[2] must comply with UK Border Force conditions that are necessary to prevent any circumvention of UK immigration controls. This includes a requirement to capture a facial biometric for each domestic passenger on entry and exit of a departure lounge in order to verify their identity. [1] http://www.heathrow.com/departures/security-and-baggage/flying-within-the-uk-and-ireland http://www.heathrow.com/departures/security-and-baggage/flyi... [2] AIUI what they mean by "common departure lounge" is a lounge accessible to international and domestic passengers without having to pass fully through to arrivals and re-checking in on another flight, but I could be wrong.