It is, indeed, a 38 S&W cartridge made by Gevelot in France. In the UK, you commonly refer to it as the 38-200. Revolvers from Webley and S&W were abundant, as issued to your armed forces. The cartridge you found appears to have the 146 grain bullet. Someone will be along shortly to shed some light on the connection between the cartridge, S&W, and its use by the British. Cheers!