Goforth shows that revolver as the No. 15 "Bobby Model". According to the book, the serial number appears on the front grip strap and also on the left side "with one or two letters whose meaning is unknown." The illustration shows a revolver identical to that pictured by the OP, and describes the markings as on the OP's gun. In 1940-1942, serial numbers 1-23782 were shipped to the British Purchasing Commission. All were in .32 Caliber. The No. 25 was in .38 S&W, but "it is unknown if any .38 caliber revolvers were purchased by Britain."
While .38 would seem to be a logical choice for the British, perhaps they took what H&R could produce, or planned to order .38's later and did not.
Jim