The published sources I've seen concur that the British Army indeed adopted a 200g LRN bullet as their Mark 1/1Z ammo in the late '20s and finally standardized the ammunition in the early 1930's. By the mid-to-late 1930s, Hague Convention concerns led them to adopt a 178g FMJ bullet, designated Mark 2/2Z. The "Z" designator indicated the use of nitrocellulose propellant instead of cordite.

Over on the Cast Boolit forums, you can see published original Brit tech drawings that we're using as a template for a group buy of molds to duplicate the 200g Mk 1 bullet, although our version will be flat-based rather than hollow-based to facilitate casting.

The 38 S&W 200g Super Police cartridge was in relatively widespread use in American police departments starting about 1930, and was virtually identical to the British Mk 1 ammunition. Sources I've seen are unable to determine which side of the Atlantic the design originated on, or whether their simultaneous appearance was coincidental.