Thanks for your question.
If a US Citizen is intending to travel to the UK for a holiday or as a Tourist, they do not necessarily need to make an application for entry clearance before they leave.
However, if a US Citizen is intending to live in the UK, he/she will need to make an application for the appropriate visa.
If you and your girlfriend are intending to get married, she should make an application for a Fiancee Visa from teh US.
Click on the following link for details of how she can make an application and a link to a copy of the form:
I hope this answers your question. If so, kindly click accept.
If you wish to discuss, please feel free to ask further questions.
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