Why Britain sourced escort carriers from North America

Were British escort carriers built as distinct British ships from the keel up?

Mostly no; many were built in the United States or Canada, then modified and fitted with additional British safety systems after arrival.

British escort carriers were not simply built at home to a uniquely British model. North American construction protected output from bombing and freed constrained British yards for cruisers, destroyers, repairs, and other theater-specific work. British yards then added modifications, especially safety systems, making production location a pragmatic logistics decision.