Jack Cross is an agent with SI6.
That organization is the Special Intelligence Section Six which was "set up after the 9/11 and subsequent 7/7 attacks to combat terrorism against the sovereign realm of Great Britain, and was answerable only to the Prime Minister of the day. This meant it had no political ties or bias. What it did have though was an executive order to handle matters with extreme prejudice." We are further told that "SI6 was the third option against terrorism" behind diplomacy first and military action second.
It is explained that SI6 was necessary because when dealing with terrorists, diplomacy "never seemed to work" and military action was largely "inappropriate as many of the terrorists lived and worked among civilians". Now run by General Bainbridge, the man who created it, SI6 sends highly qualified people to eliminate threats one-on-one.
When, in the prologue to the first recorded adventure, we first meet Cross, recruited into SI6 from the British Special Air Service (SAS), he and a colleague, Mike Flynn, are involved in a very nasty operation in Latin America working against an even nastier drug lord. It is after this mission where things did not go well that Cross decides to 'retire' from that line of work for the sake of his family and for a bit of time thereafter, we watch Flynn continue to serve SI6 in a solo capacity.
But, as Bainbridge observes to a subordinate, he considers Cross is just on "extended leave", given time to realize where his place really was, namely with SI6. "Besides what else would a man like Jack Cross do in the real world?" Cross will find that his old boss knew what he was talking about.