This map plots the settings and references in The Hitchhiker's Guide to the Galaxy
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Even more specifically, the story begins in the West Country, the quiet, rural part of south-western England where Arthur Dent lives. Later in the series it is mentioned that Arthur Dent bought his towel in Salisbury, so it is fair to assume he lived near there.
Rickmansworth is an unremarkable home counties town just outside London. Describing a character suddenly discovering the meaning of life, the universe and everything in an unremarkable café in an unexceptional town is a juxtaposition of the prosaic with the extraordinary that is typical of Adam's writing style.
Comparing the very ordinary town of Guildford with a planet somewhere near Betelgeuse is classic Adams technique. Guildford is a large town in Surrey, a county in southern England. Betelgeuse is one of the brightest stars visible from earth, located around 640 lightyears away.