To give some background you have to go back to 1940 when the UK was at risk from Germany invading after the fall of France.
Sir Edmund Ironside was made CO of the Home Guard and embarked on building a series of stop lines. Work started around June 1940 and stopped around August 1940 when Ironside was replaced and General staff thought of a different strategy.
The idea was to protect London and used natural barriers and ditches to deter any chance of the Germans using tanks like they did in France. Between the natural barriers anti-tank ditches were dug and pillboxes built to defend London from attack. When work stopped on the stop lines the pillboxes were left and over 1200 remain today.