Accordingly, a defensive line was built in the hills of southern England with the aim of arresting (or at least slowing down) the advance of an invading German army across the countryside towards London. This GHQ line was part of a wider British plan of anti-invasion defence.

Many years later, those concrete pillboxes, roadblock constructions and anti-tank defences so hurriedly built then are still abundant in the landscape around Guildford where I live.

It is both intriguing and distinctly worrying to ponder the likely course of this invasion battle for Britain which fortunately never took place.

Kenneth Macksey’s book Invasion recounts one alternative history of that event, and provides these interesting maps showing advance from a German beachhead towards London and the eventual breach of the GHQ defensive line to the south of the capital.