How do I find out if someone is a legal citizen of the UK?

And how can an american and a nigerian get married in the UK if they are not citizens of the UK?

Answer:
Most countries allow non-citizens to get married if they have both been legal residents of that country for a specified period of time.

As for your other question, I don't know. Unless you are a prospective employer, you may not be able to.
A legal citizen of the UK is someone who has a British passport.

The immigration information post by website user , MyTend.com not guarantee correctness

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