OK well you show me a law that says they must carry any particular named product!!
The Post office is simply the company that runs a number of "shops" where services are booked & paid for, Royal Mail & ParcelForce are the companies that carry the goods from A to B, they are independent companies who themselves have to comply with international law on shipping "dangerous goods" which is why they cannot over rule the CAA (civil aviation authority) over what can & cannot fly as unaccompanied freight. (and unless you have worked within aviation rules you will have no idea just how petty those rules can be).
As the company who carry the goods both RM & PF can also decide what they will or will not carry,
for those of you who don't know they did try to ban guns completely (2005 iirc) but a case was put to the ombudsman that they were the only company providing such a shipping service to the public & they were ordered to continue to provide it, but even that is not a law.
It is within their contract that a suspicious package can be inspected, and stated in their rules that any prohibited item found may be destroyed without recourse to the owner.