The story began back in 2003, when in “28 Days Later” a viral pandemic caused the breakdown of society. The movie “28 Weeks Later,” which came out in 2007, showed the same society refusing to learn the pandemic’s lessons. Then there was “28 Years Later,” last year’s installment, which was both a moving meditation on mortality and, apparently, an allegory about self-mythologization, fortress politics and isolationism, particularly in post-Brexit Britain.