Robert McGraw is an agent with the Special Branch.
That means that McGraw is both a policeman and an operative, working in the organization which in Britain straddles that border between being a crook-catcher and a spy-catcher.
He started out as the former, of course. That was almost always the way it was done with the Special Branch. He had worked for more than a few years with CID and earned his Detective Sergeant stripes many times over finding and putting away a lot of very nasty people doing very nasty things to others. Then he made the move to the Branch and continued to find and put away those who would do harm to his country.