[[WMG:''Children of Men'' takes place in the ''VideoGame/HalfLife'' universe.]]

* Between the Black Mesa incident and Half Life 2. The Combine is suppressing the ability to reproduce already. It all fits, except there's no public mention of the Seven Hour War.

[[WMG: Britain is not the only country with a functioning government.]]

* It was possible that the British government uses the "Britain stands alone" line to say that it was the only surviving nation left.
** Supported implicitly in the movie. The youngest person on Earth was an Argentinian who was killed (ostensibly by accident) while leaving a club in Buenos Aires. Doesn't sound like an anarchy to me. The idea seems to be that there are functioning governments, but they are a minority among those that collapsed.
** If Britain wasn't an attractive place to live the refugees wouldn't be trying so hard to get in, so that supports the assertion that at least mainland Europe is in big trouble.
** Theo passes by the Argentinian embassy just before he is kidnapped by the Fishes, it has flowers laid in front of it, so someone in Argentina is sending out ambassadors to other countries, or at least it's still a symbol of Argentina to people in London.

[[WMG:The sterility plague was not caused by nuclear fallout.]]
If it were, humans would not be the only species affected, yet we see many animals (cats, dogs, chickens) that have much shorter lifespans than 18 years. Whatever caused it specifically affected only humans.

[[WMG:The government of Britain is mostly comprised of [[ReasonableAuthorityFigure reasonable authority figures.]]]]
Many oppressive governments have tried to scare their people into believing the world is falling apart, and that the only reasonable course of action is to bear down to keep the outside from coming in so citizens can live in peace. What if it wasn't fiction, but the God's honest truth? How would a good government respond to a world that has actually [[CrapsackWorld gone to Hell in a hand-basket?]] The British government may simply be doing the best it can to hold things together, driven to brutality in the attempt to preserve what semblance of civilization remains.

[[WMG: England has identified the source of the fertility plague and it is communicable]]
Hence why they are trying to get the fugitives out of the country or destroy them. They want to do so before releasing the cure.

[[WMG:The sterility plague is wearing off]]
She was just one of the first.
** Pretty much confirmed by the sound of children laughing and playing at the very end of the film. Whether or not this means that all women are spontaneously fertile as suddenly as they lost their fertility, or they found a cure of some kind by studying what made Kee different from other women (antibodies? genetic mutation? exposure to chemicals? whatever it is, reproducible) and applying it to other women isn't answered. But the film's symbolism (up to her name, Kee, or rather, Key) suggests she's central to curing the plague.

[[WMG: ''Children of Men'' is a sequel to the movie ''Film/RiseOfThePlanetOfTheApes''.]]
The virus from ''Rise'' mutates so it is not so virulent and deadly, it merely removed reproductive ability. While the human race slowly exits (or at least far fewer are reproducing for now), the apes experience a great birth rate.
* So, the "II UsefulNotes/TheAmericanCivilWar" mentioned in the film was between humans and apes?
** Possibly, but more likely the apes were just biding their time as the humans killed each other off and died off.

[[WMG: The sterility plague is a side effect of the flu epidemic.]]
This seems to be implied in-universe, where it's stated the epidemic took place in 2008, with the last baby being born in 2009. While [[FalseCause correlation does not necessarily equal causation]] this seems a bit too much of a coincidence to not be related. This would also match with the WildMassGuess above that the government had identified the source but is {{withholding the cure}} for use solely on their own people. Presumably Kee is immune, if this theory holds.