British air doctrine, since the time of Chief of the Air Staff Hugh Trenchard in the early 1920s, had stressed offence was the best means of defence. British defensive strategy revolved around offensive action, what became known as the cult of the offensive. To prevent German formations from hitting targets in Britain, RAF's Bomber Command would destroy Luftwaffe aircraft on their own bases, aircraft in their factories and fuel reserves by attacking oil plants. This philosophy was impractical as Bomber Command lacked the technology and equipment and needed several years to develop it. This strategy retarded the development of fighter defences in the 1930s. Dowding agreed air defence would require some offensive action, and fighters could not defend Britain alone. Until September 1940, the RAF lacked specialist night-fighting aircraft and relied on anti-aircraft units which were poorly equipped and lacking in numbers.
What was wrong with the anti aircraft units?
poorly equipped and lacking in numbers