Why Britain expected the whole Italian fleet to answer

Why did the British expect Italian battleships, rather than only cruisers, to respond after Taranto?

Because British doctrine expected Italy to sortie its whole fleet together, including battleships, cruisers, destroyers, and air support.

British planners expected the Regia Marina to deploy its cruisers, destroyers, battleships, and air cover as a complete fleet. Against Germany, Britain more often expected a raiding battleship, cruiser, or small group. A full Italian sortie could create a much harder battle, but it also offered Cunningham the decisive fleet action he wanted. That expectation explains why the carrier escort, Otranto cruiser force, and main battle fleet were positioned as mutually supporting layers.