I assume the main advantage of going for the discriminated RCD chain
like that is reducing the likelihood of nuisance trips? Compared with
the alternative (i.e. having a standard 30mA type AC RCD feeding the
whole CU), do you think the discriminated system is worth the extra
expense?

There are 3 basic methods (with variations, of course)

1. Whole system 30mA RCD
2. Split load 100mA time delay+30mA RCD
3. 100mA time delay + RCBOs

The problem with system 1 is that it kills the lights in the event of an
earth fault, which could be very dangerous, either due to rotating machinery
or fire escape. Some would say that system type 1 is no longer allowed by
the regulations, although it isn't very explicit about it, if I recall.

The advantages of system 3 over system 2 are twofold.

Firstly, it reduces the chance of a nuisance trip as there are fewer
appliances protected by one device, leading to a lower quiescent residual
current that isn't so close to the trip point.

Secondly, it reduces the effect of a nuisance trip, limiting it to the
circuit the trip occured on.

Obviously, system 3 can be very expensive if many circuits must be 30mA
protected. You would probably err on the side of protecting more circuits on
a TT system, as the consequences of a nuisance trip on the non-RCD protected
circuits are greater, as they would trip the time delay RCD and kill the
lights.

Personally, on a TT system, I'd be tempted to abandon the whole installation
RCD and use RCBOs for every circuit without exception and an insulated
consumer unit. This provides maximum discrimination, a single box solution
and the required RCD protection for every circuit. However, it is very much
more expensive than the cheapest permitted solution. I can't remember if the
consumer unit internals require RCD protection. If they do, you'd still need
the 100mA time delay incomer.

Christian.