The BNP 303 etc etc markings are the UK Civilian Proof markings that it had to have before it could be sold into the UK Civilian gun-trade. If it doesn't have them, it would have been sold directly from the MOD surplus sales to the US ans not into the UK Civilian market first.
The "England" marking is a Pre 1968 import (to the USA
) mark. Earlier and later years had different import marks. Yours may have the Importers initials and address (for example) which was a post '68 requirement.