Group 15 is an agency inside the British Intelligence community.
It is an extremely hush-hush portion, so much so that to know about it without permission could get a person removed from existence. Whether that has actually happened or not remains unspoken but considering the many things that are slowly revealed about this organization, it is not a stretch to believe it. Certainly when we do see someone thinking about leaving their employ we find that extreme reaction taking place.
In the first instance of our learning about this Group, in a conversation between the head of the agency, called Control, and its Number One operative, Controls asks rhetorically what the Group was for. John Milton, that Number One, responded "Framing. Extortion. Elimination". Control agrees and adds, "Jobs that are too dirty for Her Majesty's security services to touch" and "From time to time Her Majesty's government needs to remove people whose continued existence poses a risk to the effective conduct of public order. The government requires particularly skilled professionals who are prepared to work on a non-attributable basis to deal with these problems". Hence Group 15 to fulfill that requirement.
As mentioned, the head of the Group is known as Control. Each of the field agent is assigned a number to denote his/her hierarchal position. As those higher up the chain fall in the line of duty, those below the deceased get their numbers moved up.
The first Number One we meet is John Milton, considered probably the deadliest man around. He was a specialist in "Cleaning", a euphemism for killing up until he decided to leave and was placed on the 'to-be-removed' list. It would be some time and some unfortunately Numbers being themselves 'cleaned' before Milton got his name removed from the removal list. [Milton has his own entry in this compendium.]
Prior to Milton's tenure, we learn about an extremely capable woman named Beatrix Rose. She, too, wanted to leave the organization and paid a horrible price for her 'betrayal'. [A set of adventures about her, both inside the organization and out, have also been chronicled.]
Within the pages of the adventures of both Milton and Rose as well as the few about Group 15 without those two, we find that the old adage about absolute power and its corruptibility holds true.