Tom Buckingham is an agent with MI-5.
He will be, anyways.
In the first of the three recorded adventures we have of him, he is serving as a Sergeant in the highly elite Special Air Squadron and has been for some time. He has the unusual codename of 'Posh Lad', given to him because, according to one friend and fellow soldier, "One: rich family. Two: he always uses the right tense. Three: he owns a fountain pen. Four: he's got name-tapes in his socks. That's pretty posh to trogs like us."
Regarding the 'rich family' part, his wealthy background was the truth but in talking about his well-to-do family, Buckingham talks about the silver spoon everyone in his lineage had supped on since Waterloo but Buckingham had chosen to "spit it out". That caused considerable friction because not only had to chosen to enter the military instead of the business world, he did so as an enlisted man and not an officer. To say that Buckingham was determined to plot his own course is putting it mildly.
While in the SAS, Buckingham had learned a lot about himself and his abilities. He found he was a born fighter and while he did not enjoy inflicting pain or injury, he definitely was good at and certainly did not shirk from doing it. "Tom had the ability to keep totally in control and still think clearly when the adrenalin was pumping. It had nothing to do with bravery. It had to do with being in control of his heartbeat when it would be natural to flap big-time, and that only came from long hours of training. Stress actually improved Tom’s performance. It always had. Even at Eton, Tom had stood out on the rugby pitch when the pressure was on."
His natural inclination to take action when doing so was needed would get him expelled from the SAS when his heroics saved lives but caused political trouble in Afghanistan. It was when he came back in England in disgust that he is given two different choices to continue his desire to serve. One was working for a powerful billionaire and up-and-coming politician with the top spot on his radar. The other was working for MI-5 monitoring that same would-be leader.