Are you talking about not RCD/RCBO-protecting the light circuit at all? If not, the problem with "not putting it on is own RCBO" would be another way of saying "put it on an RCD shared by other circuits" - and those "other circuits" could then trip the RCD, taking out the lighting circuit.Lighting does not tend to have earth leakage so no real need to put on it's own RCBO.
Kind Regards, John