That is a really nice No.5. Looks correct. Not a put-together made from No.4 parts. It's about 6 months older than mine (5/46). Made at the same Royal Ordnance Factory, Fazakerley. ROF (F). Not sure on the SN. Maybe someone forgot to add the leading zero. They would make 9999 rifles with a letter prefix starting at A then start numbering them from 1 with the next letter. The SN of my rifle is prefixed with a V.

The sling is not correct for a British rifle, but if that's what your dad put on it, I can see why you'd want to leave it that way.

The ENGLAND stamp was a pre '68 U.S. import requirement to show country of origin.
Your rifle was possibly imported by Interarmco.

The BNP stamp is Birmingham Nitro Proof. .303 is the caliber. 2.222 is the case length. 18.5 tons is the pressure of the proof round.

The proof was required for rifles sold into the civilian market by the British government.

Very nice rifle.