For tourism no, for work, volunteering, school, or stays longer than 6 months... yes. I traveled 2 summers ago and was told this by the British consulate.
US Citizens can visit the United Kingdom (tourist) without a visa. Use this link for more info. https://en.m.wikipedia.org/wiki/Visa_requirements_for_United_States_citizens
Yes yes they must and i know this because I am British.
Yes, it's nesssesary to have one in session. Here's a solution. When a traveler plans to go to London, England, UK or elsewhere he, she, they need to have a Visa on their Passport. However, it's important to let the Airport security officials to understand this and notify each citizens to have one. If they don't even if they do this without, they won't let then in. Including you. They're not going to let you enter the plane where the gate is. So it's recommended to have a Visa. That's my best advice.
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