According to Wikipedia "171st Heavy Bty [was] at 2 King Street, Port Glasgow". 171 Bty was part of Clyde Heavy Brigade*, RA during the interwar years. Again, according to Wikipedia "The regiment mobilised in the Lowland Area of Scottish Command on the outbreak of war in September 1939 with the single 171 Battery (manning 4 × 6-inch guns) under command until 407 Coast Bty joined on 31 December 1940. The coastal artillery regiments began to be reorganised from September 1940, with the Clyde regiment becoming 538th (Clyde) Coast Regiment in January 1941 with A, B, C (all formed from 171 Bty) and 407 Coast Btys. The Clyde defences consisted of:
4 × 6-inch guns
2 × 4.7-inch guns
1 × 12-pounder gun
Mid-War
On 1 April 1941 A and B Btys were renumbered 152 and 153 Coast Btys, and C Bty was split to form 154 and 155 Coast Btys, giving 538th Coast Regt the following organisation, which it retained into 1944."
However the unit in the photograph formed part of the British Expeditionary Force (BEF) which went to France and the Low Countries in 1940, whereas according to the Wikipedia account, the Clyde Heavy Regiment RA was responsible for coastal defence around the Clyde and Lanarkshire region, so remained in the UK. Furthermore, Climber says that the drill hall was "across from Greenock Cemetery". 2 King Street is nowhere near Greenock Cemetery
*At this time, a Royal Artillery Brigade was commanded by a Lt Col and was the equivalent of a regiment or battalion in size.