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Here are the display requirements for pass holders.
Whilst within British Airways Areas all staff, contractors and visitors are required to visibly display their Security ID Passes at all times, whether access to their work place is security controlled or not.
And from EN804: Displaying of identity passes:
It is a requirement under section 21c of the Aviation Security Act 1982, as amended by the Aviation and Maritime Act 1990, for ID pass holders to display their ID pass at all times whilst on duty in the restricted zone or controlled areas of airports. Additionally all Airside ID pass holders are to display their pass whilst in landside areas. This includes landside terminal areas, car parks and landside buildings.
ID passes should be displayed in one of the following ways:- attached to clothing using the clip provided on the ID pass holder
- around the neck, using the chain and holder provided
- in the clear plastic pocket of the armband style ID pass holder
- in the clear plastic pocket provided on some uniform garments
ID passes must be displayed at chest or shoulder height at all times