This is true of anyone presenting themselves at the border - the UK has the right, in most cases, to refuse that person if they don't have automatic right to entry. It just doesn't happen very often, relatively speaking, and the world isn't really like the sensationalism you see in TV programmes!

Whilst it's probably not a massively common occurrence - most people in transit won't need to cross the border - neither is your situation unique and UK Border Force will deal with a number of these every day.

The most likely reason for a UK Border officer to refuse you entry for your overnight stay is if he or she feels you are likely to disappear once through the border and not come back to board your flight the next day.

Now given:
- you've gone to the trouble of obtaining a US visa.
- you will have evidence of the ongoing flight the next day
- you will presumably have a hotel booking in London for the night
- maybe you have a hotel booking in the US
- you perhaps have details of a return flight to your home from the US
- you don't really have any reason to want to stay in the UK

you potentially have so many different things you can show the officer to indicate you aren't going to abscond that it's beyond unlikely you'll be refused entry for the night. Clearly the more of these boxes you can tick the better.

I really don't think you have any issues provided you follow the transit visa application properly.

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