Bill Tern is an agent with British Intelligence.
It is likely that the department in which he works is MI-6, aka Secret Intelligence Service, for though his employers are never designated, he does talk about working for the "Service" and considering the work he does could potentially take him anywhere in the world, MI-6 is the best bet.
He is described as "... a good-looking young man with a square, storng case and a firm, pugnacious chin. He was tall, and his long legs seemed to fit awkwardly into the narrow knee-hole of his desk. He had dark, untidy hair; the kind of hair which one associates with a rebel, and he was obviously not the kind of young man who would conform to any time-worn pattern."
From the way his boss, a very stern man with the name of Joe Rank, treats him, Tern has been an agent for a few years performing well but not incredibly so. He does have an assistant, for a while, which means he is not exactly a junior operative. Guessing his age to be around 30 is not unreasonable.
Tern is a patriot without any doubt and he would rather be doing what he is doing than anything else and could not conceive of doing it for anybody else. While he admits to himself and others that there are long periods of incredibly dullness in his job, these are made well worth it because of the times when he is forced to live life "just as hard and as full as it is possible for a man to live any life."
That is not to say that he is totally sold on the Establishment. His keen intellect and quick wit work together to let him see the foibles in everyone, including himself, and to have a laugh or two at them. A case in point is when things start to go wrong and he tells his assistant that the Tenth Parkinson Law was that "the principle purpose of a staff is to take the blame." He fulfills that requirement himself a time or two.
Tern would never be considered the epitome of any secret agent for he is a tad too glib with, well, everyone. He would be willing to give his life for his country but he would be hard-pressed to think of any one individual, other than a loved one, that he would specifically name as worthy of his sacrifice.