British submarine classes were small because each batch tested the next idea

Why would Britain build fewer pre-First World War submarines than some projected class totals suggest?

Because peacetime procurement used small evolutionary batches, while large repeated orders depended on an approaching war.

The Royal Navy used each prewar submarine generation to try a new technology, arrangement, or operational idea. That evolutionary method naturally produced smaller classes because the next design followed before a very large batch was needed. Absent wartime pressure, individual boats would also retire after roughly ten to fifteen years rather than justify enormous repeated classes. Large numbers of one type appeared when leaders believed war was imminent and immediate quantity outweighed continued experimentation. Removing that pressure from the scenario therefore reduces the expected totals for J, G, L, and related classes.