The question is: why is this system in use at some airport terminals with common departure areas, such as LHR-1 and LHR-5, but not at others, such as EDI, GLA, MAN-3, etc.?
LGW N and LGW S alongwith LHR T1 and LHR T5 are unique in that Flight Connections allows arriving passengers with onwards connections to pass through security and
not immigration before arriving in the departure lounge. Hence photo reconciliation of all domestic passengers is in force to prevent undesirables just getting on the domestic flight and getting off elsewhere in the UK without having to show a passport.
This ability is not avaialable elsewhere in the UK, indeed one cannot simply walk out of the departure lounge at these airports unescorted, whereas you can elsewhere.