Although there is no retrospective compulsion to comply with all provisions of the BCA however sometimes other works done on the building may require that the work be undertaken.
In other instances retrospectivity does apply eg glazing in doors, provision of asbestos reports for common areas etc.
If the matter has been raised as a safety issue, then the BC has been made aware of the matter and is required to address it even if that is a well reasoned rejection of the work. Is this a risk that the BC wants to bear?