I wondered why the "Z" designation and did a search but couldn't find anything.
"A Z Battery was a short range anti-aircraft weapon system, launching 3 in (76 mm) diameter rockets from ground-based single and multiple launchers, for the air defense of Great Britain in the Second World War. The rocket motors were later adapted with a new warhead for air-to-ground use as the RP-3."
Found more pics though.
This is photograph H 21141 from the collections of the Imperial War Museums.
War Office official photographer, Taylor (Lt)