British coastal defence aimed to hold invaders for the Royal Navy

The bus exercise fitted into a wider anti-invasion problem: getting defenders to a landing area quickly. Some prospective landing sites offered such poor inland access that British planners expected invaders to remain exposed to the Royal Navy. The later World War II plan similarly sought to delay an initial landing force until the fleet could intervene in strength and cut its supplies. A beachhead had to be contained before it grew large enough to threaten the country. John Bull recalls evidence of that defensive geography surviving at a Barkingside football ground, where pitch works uncovered an old tank trap when an excavator fell into it.