On the outbreak of war, the attempt at a knock out blow by bombing was expected form Germany. An Air Staff assessment carried out in April 1939 estimated a scale of attack of 700 tons of bombs every 24 hours for the first two weeks when the scale of attack would then lessen due to losses/breakdowns which would reduce the scale of attack by about 50%. It was also estimated that German air strength was double that of Britain’s (1,000 German to 500 British fighters and 2,000 German bombers to 950 British). However the chain of R.D.F stations (19 were now in place out of the 22 approved) was expected to give enough warning so the fighters could meet the German bombers over the coast. The German bombers were slower compared to the new British Hurricane and Spitfire fighters, poorly armed and would be unaccompanied by a fighter escort as they would be out of range form bases in Germany.
Air defence was still the primary measure for close defence. The Aircraft Fighting Zone was extended to cover the industrial Midlands. Anti-aircraft guns were allocated to the gun defended areas to produce the maximum rate of fire on the run up to the target – about two miles when the German bombers would have to maintain a constant course. But, at the outbreak of war, the number of guns was still well below the authorized level (for example the London and Thames approaches had 253 guns in position out of an authorized 480 guns).
Above: 1939 plan for the Air Defence of Britain. The Air Fighting
Zone was greatly extended compared to the 1935 plan and
divided into sectors
A counter offensive element was also planned with Bomber Command to organize air strikes on German industrial and airfield targets in order to reduce the scale of German air attacks.
The threat of German invasion of Britain was still considered to be negligible, due to British air and naval power. It was expected that air patrols and reconnaissance of potential invasion ports on the Continent by Coastal Command would pick up any invasion flotilla and it would be bombed or shelled to destruction before reaching the British shore.
Land Forces only needed to be kept in Britain “to man the anti-aircraft ground defences and to maintain order and essential services in the event of major and sustained air attacks”. As a result the best of the British Field Forces were sent to France as part of the Expeditionary Force with the Home Forces reduced to a token force of semi-trained troops. Output of equipment, artillery and transport was also prioritized for France.
As the nights began to lengthen in October the Cabinet did consider that a German force might slip through unnoticed and land on the British coast. As a result they asked the Chiefs of Staff to plan for a large scale raid. The resulting plan to counter this was the “Julius Caesar” plan, which actually laid down some of the fundamentals for anti-invasion planning throughout the war.
Defence Plans for the United Kingdom, TNA