Luke Pagan is an agent with MI-5.
When we first meet him as an adult he is a police officer and has no ambition to become a secret agent, if he even thought such people might exist. Certainly at the time there is no formal organization like MI-5 or MI-6; they had not yet been established. In fact, when they are brought about, young Pagan will be one of the first seconded to MI-5 from his duties as a cop. Realizing he liked the work and was good at it, he would go to make the career change.
Prior to his adulthood, at age 15 in the year 1907, we meet young Pagan working with his father as a games-keeper for a wealthy British landowner and planning to go into the priesthood with the backing of the employer as patron. A run-in with that man's youngest son that ended in blows put a stop to that as patronage was withdrawn. From this altercation Pagan would see his desires to enforce what was right would govern his chose of career. He also learned, though he might not have recognized it as such, that while honesty was the best policy, it did not always behoove someone to tell everything that one knew. This lesson would prove useful in his future.
One other thing Pagan learned during the last part of his teen years was languages. His father's friendship with the local vicar resulted in that man teaching Pagan a good understanding of French followed by tutoring, paid for by Pagan's father, in Russian. That would really be of importance years later when the needs of MI-5 for Russian speakers would be shown.
England, and London especially, in the decade leading up to the Great War was a haven for dissidents from all countries and many of those, Russian in particular, took to anarchism with the hope that if "it" was all brought down, something better would come forth. So with anarchists wanting all order disrupted and Bolsheviks wanting the upper class destroyed and Germany and Austria threatening expansion throughout Europe, the need for someone like Pagan was tremendous.