These were probably manufactured when Britain observed a policy known as Imperial Preference in the '20s and '30s, in particular. In effect, at a time of large international tariffs, Britain simply became self sufficient and traded within the Empire, which possessed all the materials required for industry. Manufactures, for the most part, originated in Britain (before our industry was hollowed out!). Raw materials were sourced from the Empire and Britain was, at that time, entirely independent
Rgds
A
Afternote: From recollection, it was the Ottawa Conference 1932 that introduced Imperial Preference as a formal policy, during the Great Depression, and this was at once implemented throughout the colonies and Dominions.