djk,
my Iron recently had a short circuit. The 16A MCB tripped and the RCD tripped very quickly. The BS1363 fuse remained fully intact.
More worryingly, I've seen the same happen with 20A diazed fuses. i.e. there was an overcurrent fault on a kettle and the plug fuse remained intact while the 20A diazed popped.
You more or less expect an MCB to trip before a BS1363 plug fuse on short circuit, considering the Time-Current Characteristic of MCB's.
It takes time to heat up a piece of tinned copper wire (albeit short), the mechanism in the MCB works by magnetism caused by the fault current on the circuit, these are pretty much instant in their action.
Also, with the diazed fuse, if they are anything like an HRC fuse, there are 7 strands of finer silver wire leading through tubes in the fuse body, this means that the fuse will blow at a lower aggregrate(sp?) fault current.
It's also done like that to ensure the fuse runs cooler.