komadori parent
Anyone can bring a private prosecution in the UK and I don't believe the Post Office has any specific additional powers in this regard.
Institutionally empowered would have been a better phrase.
While the PO doesn't have specific legislative powers to bring private prosecutions, they did have a significant amount of institutional holdover from when is was a public body.
Specifically, a close working relationship with the police and other bodies with statutory powers that went far beyond any relationship a privately owned body should have. E.g. they were given direct access to the Police National Computer System.