Do I need any documents other than my passport to travel from the United States to London England?

Check the US travel guidance website.

No. Assuming it is a US passport, the UK does not require US citizens to get a visa for a stay of up to 6 months.

I can add a few hints, though. At passport control, the immigration officer will quiz you about your visit, and it will help to have proof that you have a return flight booked, and enough money to cover you for that time. What they're mostly interested in is that you will leave when you say you will, and you won't try to illegally get a job. That's the reason for suggesting those. Have all that in whatever bag you will take on the flight with you, because you get to passport control BEFORE you get to baggage reclaim, so packing it in checked luggage is the wrong place to put it! Of course don't pack your passport either for the same reason - would you believe some people do? It needs to be on your person.

Hopefully the officer will just think you're a normal tourist and not actually ask to see any of this, but it's good to have it in case they ask.

Also take a pen because UK landing cards will be handed out on the plane and you need to fill one in. It asks for the address you are staying at so take a note of that too. You hand that over with your passport at passport control. (You'll need the pen again for filling out the US Customs form on the way home.)

Finally, a word about Customs. At British international airports, after you've got past the immigration officer (who will stamp your passport to show how long you're allowed in for), and baggage reclaim, you will come across three coloured exits. Blue is for people coming from the EU only, green is for if you don't have anything you shouldn't have with you, and red is if you have more than your duty-free limits. So assuming you don't have anything you shouldn't have, just walk through green, and almost certainly you won't be stopped. British customs officers have their own ways of deciding who looks suspicious and they're pretty good at it.

No. You can stay visa free for up to six months.

It is recommended that you bring additional documentation as on this site:

https://www.gov.uk/check-uk-visa/y/usa/tourism

You'll need proof of your return ticket and the details of your stay as well -- the border agent is going to want to see that you plan on leaving when they decide to grant you the visa at the border.

If you don't have a return ticket or your stay is longer than a few days, then you'll also need evidence of how you're paying for yourself while you're in the UK and evidence of your ties outside of the country (proof of job, your lease or mortgage, school registration, etc.)