They usually got that way when the importers had them modified to make "detective" revolvers. Cogswell and Harrison did some of those in England (they were owned by Sam Cummings at the time), and other importers had the work done in the U.S. IIRC, C&H kept the front lug, but others didn't.
Many of the .38 S&W guns were also converted to .38 Special at the same time. If that work was done in England, the British proofs will show .38 Special as the caliber and the appropriate case length.
Jim