According to my copy of Richard Arnold's "The Shooters Handbook" published 1955, the new proof mark BNP under a crown standing for Birmingham Nitro Proof, came into operation 1st February 1955 under the new Rules of Proof 1954.

The other markings on the barrel of your 8 bore such as the nominal bore in inches, disappearance of the word CHOKE and reference to service pressure are consistent with the new 1954 rules of proof.