I would say England became very much a distinctive identity and culture in at least the 13th of 14th centuries

If you mean as we understand the Nation state in the modern contacts then it would be from the French Revolution of 1789 onwards

England a bit of an aberration in many regards and the American Constitution and the European counterparts

Tried to copy the English constitutionalism but I'm that why English have a lot in common with Rome political ideals and conventions

Being un-codified