Information regarding the use of ISS facilities and services by ex members of staff (whether retired or resigned) can be found at: www.ncl.ac.uk/itservice/account-admin/accountexpirystaff/
Emeritus Professors, and other holders of honorary positions, are members of the University and thus are entitled to receive the services of ISS.
Any individual may apply for a personal sponsored JANET connection that allows the continued use of JANET. This would only be granted if the use was in relationship with academic activity, and there is a charge. In view of the proliferation of ISPs, this option is unlikely to be attractive except in so far as it might allow continuity of address.