Why mass British sloop building would not automatically produce Type XXI

Would a large prewar Royal Navy sloop program cause Germany to build Type XXI submarines earlier?

Not automatically. Type XXI depended on later wartime experience, industrial preparation, Allied air pressure, and captured Dutch technology rather than escort numbers alone.

A large British sloop program would signal stronger anti-submarine preparation, but Germany's prewar planning still centered heavily on a war involving France. The shipyards, experienced labor, and design practice needed for an immediate mass Type XXI program did not yet exist. That submarine emerged from operational lessons, increasing Allied air pressure, and access to Dutch technical work. Germany might respond instead to a larger British escort force by adjusting its own air and submarine investment, while Britain could find additional escort carriers more useful than multiplying sloops alone.