I looked up cash railways in Bucks and found this for the Bletchley Co-Op
Whilst I don't recall such a system in that one in the late 70s and early 80s I do have recollections of seeing that exact system in operation somewhere else. I would only have been very young and I have a feeling it may have been in the Co-Op in my old home-town of Langport...BLETCHLEY. Co-op. "The culmination of the Bletchley and District Co-operative Society's jubilee celebrations was reached on Saturday, when the new premises were opened in Bletchley Road... A new system of heating .. has been installed, together with a pneumatic cash carrier system." Bucks Herald, 25 Jan. 1935, p.5
• "On purchasing an item, trhe assistant would write your member's number on a check and with the cash it would be placed in a metal tube which travelled along a series of pipes to a secret place, the cash office. To me, it seemed quite magical, for a few minutes later, the tube would whizz back to the assistant with the correct change."