Harry Tennant is an agent with British Intelligence.
Technically when we first meet him he is working with British Military Intelligence but this was as the latter was morphing back into the former so it was really a difference without a distinction, at least to Tennant.
We learn early on that Tennant was affiliated with that organization because of the work he had done in the last part of the War. As he put it, "While waiting my turn to be demobbed, I had expected to spend what remained of my army career riding around the city in a Jeep helping prevent the Russians taking the sort of inch they could turn into a mile. The army hadn't quite finished with me, though, and before my papers came through I found I had been promoted [to the rank of Captain] and transferred into a section of the Intelligence Corps under a hard-faced Scot named Jekyll."
He goes on to explain, "It had been set up to investigate war crimes and I assumed someone had got wind of the fact that before the war I'd been a policeman. I suppose they thought it might give me some sort of expertise in the matter. It didn't, of course, but I soon discovered that no one else had much expertise in that line either."
It is important to note that he was not retained to handle things as famous as the Nuremberg Trials. "Our brief was to look into those cases of army personnel who may have been either the victims or the perpetrators of war crimes. And the only things we ever hooked in the piscatorial line were small fry, tiddlers from the schools of cod-faced National Socialists who had "just followed orders".
All this makes it sound like Tennant was a lower-level military cop looking into petty crimes. Perhaps that is close to what was expected him to handle. He would find the matters that came to him to take care of started small but did not stay that way for long.