Start with the definition of circuit in Part 2 of BS 7671.

Circuit. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s).

We then move on to what the RCD is doing (providing overcurrent protection, fault protection, or additional protection, and discuss which of those is necessary for individual circuits vs "circuits" in common ... and then consider selectivity. Is selectivity necessary for fault protection, overcurrent protection, and additional protection?

If the RCD is only providing additional protection, and it is acceptable in those circumstances for it to disconnect some circuits rather than just a single circuit, is that OK?

So is the busbar and the cable from the neutral bar back to the RCD a circuit? The origin of the socket, cooker, etc, circuits would be the output terminal of the MCB and the neutral bar.

So is the busbar and the cable from the neutral bar back to the RCD a circuit? The origin of the socket, cooker, etc, circuits would be the output terminal of the MCB and the neutral bar.

Chris Pearson said:

So is the busbar and the cable from the neutral bar back to the RCD a circuit?

In BS 7671, a circuit has an OCPD by definition. The OCPDs could be at the "remote" end of the circuit ... so the outgoing OCPDs for each "busbar" being a possibility ... if you agree that, in totality, they protect that busbar against overcurrent? If not, the DNO cutout perhaps?

Certainly the devices would have to be in a circuit (distribution circuit or final circuit) somehow, because they are required to be protected against overcurrent (following manufacturer's instructions).

gkenyon said:

The OCPDs could be at the "remote" end of the circuit ... so the outgoing OCPDs for each "busbar" being a possibility ... if you agree that, in totality, they protect that busbar against overcurrent? If not, the DNO cutout perhaps?

I see what you mean. So the circuit in the CU is from the main switch via the busbar to the outgoing terminal of the MCB. We must include the MCB in the circuit as the OCPD.

Back to the OP and Alan's response: my preference is for an all RCBO board (save for any distribution circuits, or dare I say, to a PV array), but I am fortunate inasmuch as cost is not my primary concern.