They had a lot of faith in one of Churchill pet project the Parachute and Cable system, and the Z Battery...and shot to pieces in short order. Airbases are always heavily defended with AAA.
From the wiki
On 5 June 1940, after the British Army had been evacuated from Dunkirk and the Battle of Britain was about to start, the 1st AA Division comprised 45 4.5-inch, 39 3.7-inch and 26 3-inch HAA guns, with three 3-inch, 19 Bofors, three twin Vickers and 185 LMGs in the LAA role, together with 240 90 cm searchlights.[18] On 11 July, the division's guns were disposed with 92 defending London, 28 at Slough, 4 at Hounslow, 4 at Stanmore, and 34 others dispersed to VPs.[19]
While the Luftwaffe attacked RAF airfields, only the guns of the 48th AA Brigade in south-east London were engaged. On 1 September, over 200 aircraft attacked Maidstone, RAF Biggin Hill, RAF Kenley and Chatham: the guns of the 1st and 6th AA Divisions broke up the attacks but Kenley and Biggin Hill were badly hit. The following day a raid up the Thames estuary reached the edge of the London IAZ and were engaged by the 26th AA Brigade. Between 11 and 15 September, massed raids approached London, but running battles with RAF fighters broke up most of the raids before they reached the IAZ, where they were engaged by the 48th AA Brigade.[20]
so not a lot of light AAA that would be effective for low level fighters