I'm not sure that such an exemption from holding a FAC exists.
People in business as carriers, RFDs or their servants - I think these folk may in the course of their trade possess shotguns without holding SGC.
I think Sec 11 (1) below is usually taken to mean possession by someone who is with the certificate-holder and who is under the certifcate-holders immediate instruction, and attendant on their current sporting purpose. I'm not sure what the case-law is, though.
Firearms Act Sec 11 (1)
A person carrying a firearm or ammunition belonging to another
person holding a certificate under this Act may, without himself
holding such a certificate, have in his possession that firearm or
ammunition under instructions from, and for the use of, that
other person for sporting purposes only.