Capacity was the decisive constraint

The E-boat was a real and costly threat, but Britain already had methods for countering it. The Royal Navy had tactics, radar, committed crews, and a numerical response comparable to its anti-submarine effort. Pre-war planning assumed that war would come later, that France and Norway would remain in the fight, and that Britain would avoid facing Germany, Italy, and Japan simultaneously. Those assumptions collapsed, leaving too few boats and people for commitments spread across the world. The resulting losses demonstrate overstretch rather than the absence of an effective method. The wartime Royal Navy remained a capable institution under exceptional strain, and it still suffered serious losses.