Welcome to the Metrozone. To give it its full name, the London Metrozone. Twenty-five million people, set behind a wall of concrete and wire a hundred miles long, reinforced by automatic guns and watchtowers. It has the economy of a prosperous industrialised nation, its citizens come from every corner of the planet and it’s the last city in England.

Things look the same, but different: the Houses of Parliament – disused but safe from flooding behind a massive dam. Marylebone station lies dormant: no more trains to the Midlands, because the Midlands are an irradiated wasteland. Buckingham Palace is still at the end of the Mall, but it’s flagless. Regent’s Park is now home to thousands of refugees in their converted shipping-container houses. England, as a country, has ceased to exist. The only part of it remaining is the Metrozone.

What happened? Armageddon. But the brief, world-changing years of nuclear terrorism are a fading memory. The city remains.

So why pick on London? I mean, what’s it ever done to me? Do I take perverse delight in trashing my capital city, threatening it with flood, fire, war and disease, wrecking the national monuments and destroying millennia of history?

Yes. But that’s not reason enough. Okay, setting a series of novels in a post-apocalyptic London is an obvious choice, simply because it’s the biggest and most well known city on these islands. It has iconic buildings and internationally recognisable landmarks, in a way that Coventry, Aberystwyth or Motherwell don’t. St. Paul’s, the Gherkin, Battersea power station, Trafalgar Square, Tower Bridge: all are instantly recognisable from thousands of books and hundreds of films by people who live half a world away and will never see London for real.

What puts the smart money on London surviving while Bristol, Birmingham and Bolton empty is a series of genuinely coherent arguments. Honest.

First up: the M25. It’s not quite round. It might even be in the shape of a demonic sygil. But it does go all the way around most of Greater London, encompassing a vast area of land – not just houses, but factories, warehouses, Heathrow airport, the Tube, and lots of parks. It’s ready-made clear-cut, just waiting for a high wall and coils of razor wire. With all the UK’s armed forces withdrawing to either London Metrozone or the Glasgow-Edinburgh Axis, they have to be given something to do. That something is throw up a ring of steel and concrete, and stop the Armageddonists getting through.

Secondly: London is the easiest place to get to in the whole country. It sits at the end of several major motorways, rail routes and it even contains the world’s busiest airport. That makes it not just a magnet for displaced people inside Britain, but also from abroad – like my protagonist, Petrovitch.

Okay, so we’ve crammed half the country’s population (plus a few extras) inside the roughly 1600 square kilometres of the M25. That’s 15,000 people per square kilometre. But that figure wouldn’t put it at the top of a list of crowded cities – for urban municipalities, which seems to be the fairest measure, it’d come in tenth, ahead of Buenos Aires but behind Jakarta. For comparison, New York City has a density of 10,000 people per square kilometre. Delhi is home to nearly 14 million people, a density of nearly 32,000 per square kilometre. The Metrozone isn’t beyond the bounds of possibility. In fact, it’s doable. There is even room for more. Which is scary.

Then consider the economy: London currently has a GDP of around $500 billion. That’d make it roughly the 20th richest nation on Earth. I have, of course, thrown the global finance system into turmoil and laid waste to entire countries, so some of those figures are going to be wrong. But London doesn’t rely on its hinterland for its economy – in fact it’s the reverse. All the money generated in the Metrozone will get spent in the Metrozone. There is, quite literally, nowhere else to go for a night out unless you get on a plane.

But what it boils down to is this: what is it that we expect of a city – our capital city? We expect it to be there. For us. So when Petrovitch says “I love this place”, that’s exactly what he means. Not that he’s having a good time, or that it’s easy living there, or that it’s perfect. It means that it’s taken him in, sheltered him, and kept the barbarians out. The city loved him first.