Flying throughout the Battle of Britain with No. 501 based at Gravesend or Croydon, Lacey became one of the highest scoring pilots of the battle. His first kill of the battle was on 20 July 1940, when he shot down a Bf 109E of "Jagdgeschwader 27". He then claimed a destroyed Ju 87 and a "probable" Ju 87 on 12 August, a damaged Bf 110 and a damaged Do 17 on 15 August and a probable Bf 109 on 16 August. He destroyed a Ju 88 and damaged a Dornier Do 17 on 24 August and shot down a Bf 109 of "Jagdgeschwader 3" on 29 August. He had bailed out unharmed after being hit by return fire from a Heinkel He 111 on 13 August.