Following the successful invasion of France by the Germans in 1940, Britain commenced significant preparations to defend the country against the anticipated assault.
The first line of defence that would have been encountered was known as the coastal crust. This process fortified vulnerable coastline with coastal artillery batteries, pillboxes, minefields, barbed wire entanglements, anti-tank blocks and underwater obstacles. Coastal piers were partly demolished and some lowland areas were flooded; all these measures were intended to delay the enemy's advance and channel them into certain areas. A detailed and animated description of the defence of Walberswick on England's east coast is available online. Anti-tank mines are still found occasionally, as this media report from Suffolk describes.
Further inland, stop lines and anti-tank islands were devised, enhancing natural features to create long barriers. Anti-tank obstacles and pillboxes were constructed, bridges were prepared for demolition and mines laid.
In total, about 28,000 pillboxes were constructed and 6,000 still survive; one example is the Essex lozenge pillbox, shown in the photo, which is constructed through a sea wall. Pillboxes would often have a slit trench associated with them and most of the soldiers would fight from the trench. Primary defences would be manned by the regular army, but secondary defences and anti-tank islands would often be defended by the Home Guard. Grenades were found at a pillbox in Norfolk, as this report describes.
By June 1940, the Army's strength was 1.65 million people, consisting of the Regular and Territorial Army that existed before WW2 and the additional numbers gained from conscription. By modern standards, the size of this force was enormous; they were relatively well trained and equipped.
The full time armed forces would be less likely to deliberately bury or hide explosives than the Home Guard, but ordnance could have been discarded during training exercises or left behind in temporary stores.
The Home Guard (originally known as the Local Defence Volunteers) was operational from 1940 to 1945. It consisted of up to 1.5 million unpaid people, aged from 17 to 65, who were not in military service but wished to defend their country against an invasion.
The official expectation had been to create an armed police constabulary which, in the event of an invasion would have a relatively passive role, due to their limited training and equipment. Intended tasks included observation of German troop movements and guarding places of strategic or tactical importance.
Many volunteers had a military background and a more active role evolved so that they would have been responsible for delaying and obstructing German forces through any means possible in the event of invasion. The photograph shows members of the Home Guard practice the defence of a road block.
The relatively informal nature of the Home Guard meant that control of weapons and explosives was not as strict as could be expected in the regular army. As such, discoveries of their weapon caches still occur, as these media reports from Cambridge, Hereford and Lowestoft demonstrate.
Auxiliary Units were created in 1940 to wage short term guerrilla warfare against an occupying German army. It has been estimated that 3500 people were trained in assassination, demolition and sabotage. They worked in an operational patrol of 4-8 people, ostensibly as part of the Home Guard. It is thought that 534 operating bases (OB) or hide outs had been secretly constructed by the end of 1941, holding more powerful weapons and explosives than were made available to the Home Guard. The expectation was that each OB would have 48 grenades, 3 cases of sticky bombs, 2 cases of phosphorus grenades and half a ton of explosives.
It is possible that remnants of their weapons and explosives caches could be discovered during construction work in rural areas, as not all was collected following the demobilisation of Auxiliary Units. Indeed, it was said that one person handed in their explosives and ammunition twenty years after the end of the war, which included 550kg of explosive.