Looking at what documentation I can find it *seems* as if *all* cable supports for exposed wiring have to conform to the same spacing requirements as pre-18th cable supports did.
Applying this to using metal clips instead of plastic ones for exposed cables isn't unreasonable but do you really have to put supports so frequently where cable is in trunking?
It would mean that trunking would have to have supports every 250mm if it had 1sqmm or 1.5sqmmm T&E in it. If one was using external clips round the trunking this would look pretty silly and makes the trunking rather pointless from the point of view of appearance.
I seem to remember someone, somewhere saying that not *all* cable supports needed to be fireproof, just enough to prevent the cable falling and causing a hazard but I can't find anything like this now.
Apart from anything else it makes running a cable rather expensive!