The United States is a monolithic “nation” as well as “state”. The United Kingdom is a single “state” made up of three separate “nations” (England, Scotland, Wales and Northern Ireland). England continues to remain separate country from Scotland, and this contributes significantly to criminal and civil law, ownership or tenancy of land, and various privileges of nobility. In the past, at least, it also affected ordinary citizenship. The United States, by contrast, does not see quite such dramatic changes from each “sub-national” state to state…
More analogous to the Scotland/England–United Kingdom relationship would be th the various Indian tribal nations, that are culturally distinct, and retain a distinct limited sovereignty under the federal Constitution of the United States. England and Scotland retain certain rights and privileges as separate nations under the “unwritten” constitution of the United Kingdom.
We don’t draw the line at Cromwell because he interrupted the monarchial line; we draw the line at Cromwell because prior to his New Model Army (on which the subsequent British/UK army is structurally based), the English (British, whatever) army was structured after the Roman model.