... he mentioned that there would probably be borrowed neutrals upstairs and that if this was the case then he would have to put the lights all on one circuit. I don’t doubt what he’s saying but wondered if someone could explain why. I’m just wondering why it’s ok to have separate upstairs and downstairs lighting circuits with borrowed neutrals on a cartridge fuse system but not on a rcbo or mcb consumer unit.
A 'borrowed neutral' is when one or more lights uses the neutral of another lighting circuit, rather than it's own. This was often done for convenience, particularly with landing/hall lights.
The issue is with RCDs or RCBOs, which rely on identical currents flowing in the live and neutral wires of a circuit (if they differ, it trips). If two lighting circuits are connected to different RCDs/RCBOs, a 'borrowed neutral' will mean that some of the neutral current will go through the 'wrong' RCD/RCBO, causing the devices (probably both of them) to trip.
If both lighting circuits were connected to the same RCD, the borrowed neutral would not be a problem (there would be no reason to combine them into a single circuit) - but that is considered bad practice (and may be non-compliant with regulations), since one RCD tripping would take out all the lights in the house. If you are going to have RCBOs (effectively a separate 'RCD' for each circuit), whilst there is a 'borrowed neutral' there would be no solution other than combining the two lighting circuits into one.
The proper solution is attend to the wiring to get rid of the borrowed neutral, and then to have the two lighting circuits protected by different RCDs or RCBOs - which may or may not be easy to implement.
By no means all old wiring has 'borrowed neutrals'. Unless the electrician did tests and confirmed that you do have the issue, it could be that the problem is not going to arise, anyway.
Kind Regards, John