I think radiohead's point is if a final circuit has a N-E fault then you will probably trip the incomer RCD as the SP RCBO will trip but not isolate the neutral fault.There is no requirement for a protective device to isolate the neutral under fault conditions, regardless of earthing system type.
So it is not necessary according to the regs as such, but it is far better to have a N-switching RCBO if you can. I guess it indirectly comes under the requirement for selectivity, etc (off hand don't remember the numbers).
Not that many do unfortunately, but at my last count Wylex/Crabreee/Fusebox (the miniature version) RCBO for consumer DB do switch neutral.