This model is in two parts, one piece for the hull and another for the gun. The gun can be rotated, but not elevated.
The crew figures in the product photo are from Peter Pig, and are not supplied in the model files.
The Birch Gun was a British InterWar experiment with self-propelled artillery, utilizing the chassis of the Vickers Medium Mk.C to carry an 18 pounder gun on a rotating mount, giving a 360 degree field of fire. The gun could be elevated past 90 degrees, which would (in theory) allow it to be used in an AA role as well.
The concept was tested extensively, but in the end the British Army chose not to proceed with the idea, and would not try again with self-propelled guns until WWII.