This is the apprvoved list you need to use when incorporating companies: https://www.gov.uk/government/publications/nationalities. However this was only introduced a couple of years ago. Prior to that I think it used standard ISO country codes: https://www.iso.org/iso-3166-country-codes.html
I expect that some time in the past when it was just paper forms it was just free text and there may not have been any validation.
There is a post on another forum that suggests at some point the data on hand was going to be "cleansed". I'm not sure if that ever happened or will ever happen. There will be records where the country listed no longer exists on the official list and can't be mapped one-to-one etc.
Jurisdiction (the enumeration you posted above) is a different beast.