It sure is a puzzle.
City evacuated, presumably uninhabitable but mercenaries able to roam around freely with out any health impacts. Why wouldn’t people return?
From this I’d assume that while there is localised radiation that can be avoided (tube stations) there is a bigger risk. It may be that London was seeded with hundreds of Dirty Bombs in unknown locations. After several are detonated over a period of months the Government decide to evacuate the entire city. This would explain the initial panicked exodus (Waterloo) and then a more staged, compulsory evacuation.
Even if the city was irradiated/risky why is there no military presence to protect assets, locate bombs, clean up etc. Even in the most dangerous areas autonomous robots exist and could patrol the streets, likewise with drones. Helicopters would be on permanent patrols. Rapid response units would likely chopper in to deal with any incursions and breaches of the containment zone.
My only thought here is that there simply isn’t a government, as such, left, nor a military. Whatever conflict inspired the bombing in London may have been the result of a larger conflict over the country culminating in the bombing and collapse of the government forces. As plentiful as London is, right now, for those outside the containment zone, it is not a priority to either protect or exploit.

but never could find on google the definition of it


