Why Britain expected French submarines to cover the Mediterranean

Why had Britain not built up a larger submarine force for the Mediterranean before World War II?

Treaty restrictions limited British submarine construction, and planners expected French submarines to remain available, so they assigned British boats elsewhere.

British planners expected the French submarine force to help cover the Mediterranean. Treaty limits restricted how many submarines Britain could build, which made duplicating that allied capability unattractive. British boats could instead serve in other theaters. The question also recalls the wartime crash construction of U-class and later V-class submarines to meet Mediterranean needs.