What nationality are you?
For how long would the person be visiting?
As a US citizen the person would be entitled to receive a 6 month visa upon port of entry at customs automatically. It is technically discretionary though, so it would be sensible for the person to have evidence of travel insurance and of the flight booking for their return journey. Possibly also evidence of the money they have available.
US citizens are generally only refused where customs think that they cannot support themselves during their stay, or are a risk of becoming an overstay or will commit a crime.
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And, why do you need a VISA? Can anyone get one? Can someone
get one ahead so they know they can get in? Can they travel without
VISAs are used to monitor and limit the amount of both workers and non-workers in the UK.
You can get a visitor's visa subject to being able to support yourself and returning before the end of the expiry of the visa.
You can apply directly to the UK embassy to obtain a visa before you travel but provided you meet the requirement I mentioned previously then you should be fine to travel without a visa because you will be granted one at UK customers, but it will only be for 6 months maximum.
If you wish to apply for a longer visa (eg. 2, 5 or 10 year multi-entry visa) then you must do this directly to the local authority.
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