Admittedly I don't know the setup at Manchester, but have you considered that this might be a requirement from a Control Authority, such as Customs, rather than Security?
At certain airports, Customs require all staff to exit through the Green Channel (NOT Blue) so that they can be examined as they pass to landside. That's not giving any secrets away - there are notices about which are/can be read by millions of passengers.
Cheers,
The Odd One