Ten days ago I duly arrived at T3 to find that all of us had to pass through a retina scan
This procedure has been mandated by the DfT, NOT the airport and is not actually a Retina scan but a facial recognition scan.
even those who were not moving onto another flight.
I assume you're referring to pax terminating at MAN !? These pax would simply leave the building so not sure why you say these people had to be scanned.