Your description and the photo makings indicate a WWII Lend-Lease "Victory" model revolver provided to the British Government.

The BNP means "British Nitro Proof", the 7 12 Tonnes is the chamber pressure rating for the .38 S&W cartridge, and the shortened barrel and added front sight looks like the work of Cogswell and Harrison, a firm that converted thousands of said surplus revolvers after WWII, marked them in accordance with British law, and exported them.

This model was the wartime production of the Military and Police model, or what became known as the Model 10 after 1957.