On checking: You're correct that the royal mail and the post office are separate organisations. They split in 2012. At the time of the majority of the active development of the horizon scandal, they were the same organisation however.

I would still want to apply the same test - not just demonstrating a notional or paper loss, but that something has actually been stolen and acquired by some other party. This was one of the signal failures with the horizon scandal: that it was simply a bookkeeping error and they were unable to show beyond that any theft or loss on their part or gain by another party.