Or is it assumed that all licenses with an IR issued post 25/4/18 are automatically PBN.
Not just those issued post 25/4/18 - all IRs include PBN privileges.
FCL.605 IR-Privileges
(a) The privileges of a holder of an IR are to fly aircraft under IFR, including PBN operations, with a minimum decision height of no less than 200 feet (60 m).