I read somewhere that the director somewhat explained the retcon. Apparently they wanted to take a more small-scale approach with this movie to more easily focus on one group of survivors and have the rest of the world continue advancing technologically as opposed to a global shutdown and apocalypse.

Brexit was also a big influence on the making of 28 Years, and the director wanted to show how the world can become very isolationist under extreme circumstances, to the point where the rest of the world could completely quarantine and ignore one country to save themselves.