Following a discussion in another place I realise I am confused by this.
Consider premises with PME earthing which are currently suffering from a broken neutral wire with no adequate earth remaining connectd. Neutral swings to live potential. This is a well known theoretical problem if rare and unlikely in practice.
Consider a potential electric shock when touching live (or neutral) while being earthed by some extraneous earth, say in the garden for instance. AFAICS a double pole 30mA fast RCD will protect you but a single pole RCBO won't.
Further consider a potential electric shock from touching the case of an electrical appliance, connected to the protective earth of the premises, while connected to an extraneous earth. I don't think either of the RCD devices will protect you as they don't switch the protective earth connection. But will the RCD be triggered? And does it depend on whether the current comes predominantly from the index premises or a neighbouring one?
Am I over-thinking this?