The island in the novel, is inspired by the hilltop town of Hampstead in London and its famous parkland Hampstead Heath. In the book, Self describes a future England which has been inundated with rising seas, leaving Hampstead as the only remaining part of London. The inhabitants of this area, unaware that the drowned city of London is so close by, know their island as Ham. This abbreviation of modern-day place names is common throughout the book, with other cities such as Luton, Birmingham and Nottingham surviving into the future as 'Lut', 'Brum' and 'Nott'.
The geography of the island, illustrated in a map at the start of the book, bears close resemblance to the modern areas of Hampstead which inspired it. Sea-level rise animations by Google Earth, show that if London was indeed submerged under a 100m sea level rise, the high hills of Hampstead would indeed create an island of the shape illustrated in the book. Self writes, "...the Heath...this peculiar island, a couple of square miles of woodland and meadow set down in the lagoon of the city."