After World War I, "the war to end all wars", Britain (along with many other nations) had wound down its military capability. Germany was not considered a threat during the 1920s, but the situation changed radically when Adolf Hitler came to power in 1933 and withdrew Germany from the League of Nations and the Geneva Disarmament conference.
Even in the mid-1930s the Royal Air Force's front-line fighters were biplanes, little different from those employed in WW I. The Re-Armament Programme enabled the RAF to acquire modern monoplanes, like the Hawker Hurricane and Supermarine Spitfire, such that sufficient numbers were available to defend the UK in the Battle of Britain in 1940, during the early stages of World War 2.
The British Army was supplied with modern tanks, howitzers, etc and the Royal Ordnance Factories started to produce munitions on a large scale.
Government owned, so-called, Shadow Factories were established, to supplement the efforts of the and private industry and Agency Factories to supplement the efforts of the Royal Ordnance Factories.