No {{united_kingdom}} won't work. You need to use capital letter K in "Kingdom" for some reason.
This will work: {{United Kingdom}} or {{united Kingdom}}.
This will not work: {{United kingdom}} or {{united kingdom}}
By the way,
{{USA}} = United States
{{United States}} will not work.
{{Macedonia}} = FYR Macedonia
{{FYR Macedonia}} will not work.
{{Netherlands}} = The Netherlands
{{The Netherlands}} will not work.
{{Bosnia and Herzegovina}} = Bosnia-Herzegovina
{{Bosnia-Herzegovina}} will not work.
{{Trinidad and Tobago}} = Trinidad-Tobago
{{Trinidad-Tobago}} will not work.