B16 MCB feeding a 13amp Fuse-Connection Unit on a 2.5.sq.mm cable.
This very common household circuit arrangement surely must offer a satisfactory discrimination - if not, then it would not have been so widely adopted in househod wiring, to feed Fridge Freezers, washing machines, immersion heaters, and space heaters etc....
Only in the sense that providing there is never a problem that would blow the fuse, the trip never fires either.
As fuses almost never blow, one could equally validly conclude you don't need them. That is not the usual practice however.
-except on the continent, where C16 radials are the norm.
Of course, iI agree in the case of a resistive load like an immersion heater there is no inrush, and the load is either correct, or dead short because the insulation jacket has pinholed, and it matters not one iota that there is a race to see if the trip fires or the fuse blows first. For an immersion I have no problem with a B16.
Socket circuits are a different matter -larger fridge-freezers - even with no additional load on circuit, can have a perfectly normal inrush transient exceeding 30A - a near lower limit B16 is then teetering on the edge of a nuisance trip. Add a few more loads to the same circuit, and it is not a good idea at all.
Inrush surges on TVs and PCs with CRT monitors can be in the 50-70A range, and once again, if such loads are to work reliably from a 16A breaker, a C series would really be preferable.