I saw this post somewhere else, apparently written by a postman so maybe he knows what he’s talking about:
“It's not just a royal mail policy as its rules brought in by the Civil Aviation Authority. You will find similar restrictions in place by other couriers. It's the flying of the items that is the issue. They would be fine going by a road ie a 48 hour service but i believe the same rules still apply but the checks are not as stringent if its not going near an air craft. Non flyable items used to be diverted to a road service but the CAA rule change now means that prohibited items must be seized and disposed of when discovered.”
If this is true, then it could potentially apply to all carriers, not just Royal Mail, especially if the parcel needs to go by air freight. I guess it implies that all packages have to be X-ray scanned prior to going by air freight, whichever carrier it is.
It doesn’t explain though why RM apply the CAA rule also to road freight. As the poster says, there's no sense whatsoever in applying a CAA rule to road freight. I don’t know whether other carriers do the same thing, or indeed whether they’re physically capable of scanning all road freight parcels, as RM possibly are.
It does seem to argue for using the cheapest, slowest service you can find, as that means there is no chance of your item going anywhere near an aircraft and, at such a cheap price, your carrier probably couldn’t afford to scan your package even if it was daft enough to want to.