Australia just did that.
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I imagine that it is the same universe as Children of Men. I heard the same theory, the rest of the world is normal, Britain just decided to be that way and become even more of a dysfunctional shithole.
They finally completely gutted the NHS and that's why the plot is the way it is.
There was nothing wrong with anyone biologically, it was just a hyper South Korean style population collapse would be a hilarious twist.
Also why it makes sense in that case that the woman that is pregnant is an immigrant from ME or Africa I guess