The Nature of Civilisation
At its heart, 28 Days Later is a philosophical work about the nature of civilisation, slowly worked through from the lowest end expectation to the highest. It may seem weighty fare for a zombie flick, but it's hardly the first time the genre has been used for discussing the nature of life: Zombies work very neatly for this as a refutation of life!
The film begins with Jim alone, the sole man, to introduce us to the premise and lead us in. He soon meets Selena, whose creed is "Survival is good as it gets". That is: She rejects the idea of civilisation wholesale — Yes, she might argue, it's nice if you can get it, but in the end it's a front. Her premise seems sound, but is quickly demolished by the arrival of Frank and Hannah. It's not a coincidence that we meet two here; Frank's take on the nature of civilisation is, after all, human contact. It's in the meaning between people, and while it may not need many, it suggests that one person living alone is a waste. You may be surviving, but for what? Ultimately, Selena is forced to accept the truth of this. She - word for word - admits that survival may not be as good as it gets. That's the key admission: Life can be better than mere survival.
The dark side to this is at the other end: The soldiers. Here is the idea of civilisation as something more than that — Civilisation as institution. Again, it's not a coincidence that this bridge of the story begins with the army and is the first 'normal' human institution we encounter. That is the theme they are built to express. While human connection is fragile, ebbing and dying as people are born and die, the institution is a thing that endures. Such is the philosophy espoused by West, who has to stop a soldier from suicide because, as he notes, "What's the point?"
This is a key moment in the third act: It is used to suggest the motivation of the attempted rape of Selena and Hannah. Without reproduction, without something lasting, there's no point in this philosophy to existing at all. But the goal used to seek it likewise betrays the problem in the philosophy: It's dehumanising. Selena and Hannah are quickly reduced to playing pieces in the pursuit of the larger goal. Jim, being an obstacle to this, is quickly removed.
We can also see this in the opening scenes, which became praised and admired for their iconic depiction of an abandoned, post-apocalyptic London. While Britain is not only London, London is in many ways a central, unifying part of Britain; it's the seat of national government, and politics, the heart of national finance, and a key centre of national culture. In his walk around London, Jim encounters the central representations of these institutions in British life — the Houses of Parliament, the Bank of England, St. Paul's Cathedral, and so on — and they are all, tellingly, not destroyed but empty, devoid of 'civilization'. We tend to think of these institutions as powerful, omnipotent forces which heavily impact on our lives while we exercise little control over them in return, but the truth is these institutions are utterly dependant on people coming together in order to function, and without this human contact are little more than empty buildings slowly crumbling with the elements.
In the end, the film essentially opts for the Three Bears logic of civilisation: Human contact, somewhere between mere survival and the institution, is at the heart of civilisation.