Putting 1960s diesel technology into a U-class submarine

What would the Royal Navy build with access to 1960s-quality diesel-submarine technology in the late 1930s?

Build a much improved U-class submarine alongside the larger boats already occupying the bigger yards.

Shipyard capacity matters alongside tactical performance. Large yards could continue producing bigger boats, while smaller facilities built U-class submarines. Later battery, machinery, and density improvements would make the compact design much more capable without losing its suitability for the Mediterranean and training duties. The result adapts advanced technology to an existing production system instead of replacing every design at once.