Americans don't need a pre-arranged visa to go to the UK, and vice-versa. If you're going to work, and intend on getting into the country with a tourist stamp, then you'll be breaking the law, and risk being banned from the UK. So no, to answer your question, you don't need a visa to go to the UK. You can just jump on a place and go there to visit. But if you intend to work, do it legitimately and get a proper work permit and visa.