Initially known as Anti-Aircraft Regiments, they added Heavy to their title when the Light Anti-Aircraft units were formed early in the war. Originally armed with the 3 inch 20 cwt Gun, they soon rearmed with the 3.7 inch Gun (mobile & static) and the static mounted 4.5 inch and 5.25 inch Guns.
“Diluted” regiments
From early 1943 some Heavy Anti-Aircraft units had African Pioneer Corps personnel to replace British Other Ranks. See the link for more details.