One reason that the first modern writers in this field used London as their setting for civilization's collapse could be the grip that the legend of the “London Stone” had on literary minds, in the way that it linked the distant past and distant future. Note this ancient saying:
So long as the stone of Brutus is safe, so long shall London flourish.This carved, oolite limestone block was once the most celebrated remnant of London’s ancient origins. Local lore has variously claimed the Stone to be from the settlement built by the legendary founder of London, Brutus of Troy; or maybe a Druidic stone from a worship ring on Ludgate Hill; or perhaps the city’s milliarium, a milestone used by Romans for measuring road distances.
More likely is that the Stone simply broke off from a long-forgotten Roman monument near Walbrook, the rest of which might still rest a dozen feet under the pavement of Cannon Street.
The Roman city of Londinium itself lay mouldering for more than a century after the Romans scooted in 410 AD. The victorious Saxons preferred to live just to the west, outside the walls, in a town called Lundenwic. Apparently they found the Romans ruins too spooky for comfort, though they erected a cathedral there. Occasional discoveries of mammoth bones during Saxon times could have led them to believe that Londinium had been built by giants.
The name London Stone first appears in property records from the Tenth Century. The name of London’s original mayor, Henry FitzAilwyn de Londenstane, signified that he was born in the neighborhood of the Stone’s original location. At the time, circa 1160, the stone lay smack in the middle of Candlewick Street.
In 1450, the invading rebel Jack Cade thought that striking the stone with his staff and proclaiming himself Mayor of London would get him a quick promotion. Cade was wrong about that, but he did help trigger the War of the Roses and earned a place in Shakespeare’s Henry VI, Part 2. (Side note: In the play it's one of Cade's henchmen, "Dick the Butcher," who utters the familiar line "first thing we do, let's kill all the lawyers.")
Since then the Stone earned mentions in works by Shakespeare, Kipling, Dickens, and Blake.
While embedded in Cannon Street, a constant annoyance to operators of wagons and carts, the London Stone carried more than historical significance. As one example, the spectacle-maker’s guild took pairs of unlicensed spectacles out to the Stone to be ceremonially smashed. Later it was grubbed out of the street and in 1798 mortared into a wall of St. Swithin’s Church. German bombers flattened that structure during the Blitz but the London Stone survived the attack, as rocks will, and can be seen today. It's behind a sheet of glass, which is behind a protective grill, which is in a wall, at 111 Cannon Street, London EC4.