A black operation or black op is a covert operation by a government, a government agency, or a military organization, which often performs clandestine, illicit and/or ethically questionable activities and projects.
In 28 Days Later
After the Original Outbreak of the Rage Virus in mainland Great Britain, a ruthless black-ops force went to Scotland and set up camp in rural Scotland. The black operation intended to capture, study and try to modify and weaponise the Infection, capturing Infected from the area surrounding their camp for their experiments with the Virus, and, whenever they came across survivors, infecting them for use in the experiments. The black-ops in Scotland were apparently also in contact with Captain Stiles and aware of his hunt for the Manchester Three.
After capturing Selena, Clint Harris and Derrick and detaining them, the black-ops' camp was threatened by a forest fire they had used to try and drive Infected out to capture, and planned to evacuate to another post once the fires reached the black-ops camp. However, before they could leave, the black-ops were attacked and massacred by their captive Infected in the chaos after Selena released them, and thus the black-ops outpost in Scotland was presumably wiped out.