Where do I say a company is a foreign subsidiary or what currency it reports in?

Not on Company Structure. Entity kind, jurisdiction and functional currency are edited only on Settings → Organisation → Companies & entities. They decide what each row IS: a foreign company's registry details show on its workspace instead of Companies House fields, and UK-only provisions (ICO fee, CITB levy, ONS surveys, UK corporation tax, group relief, the operator licence…) show a Not applicable badge with the reason, e.g. 'Registered in <country>'. A person can override any provision there — Applies or Does not apply — and the override wins both ways. A failed read treats every company as a legal UK company, so an error never hides a company.