If you are traveling with a US passport, you can come to the UK as a visitor for 6 months without needing a visa. However, if the UK immigration authorities think you are coming to work or study, or if they think you are going to stay more than 6 months, they can refuse you entry. They are only likely to do this if they have some reason to suspect that you are not a tourist. So, if you are going to have some red flag in your profile, you would want to bring things to 'negate' this.
Jun 22, 2015 - Answer 1 of 3: Travelling from UK to the USA in October. * I know I will need to register for. Customs form, landing card, details of hotel address in USA? * Anything else? A sample of a completed landing card is provided here. Note that, “port of last departure” means the last place you got on a plane before arriving to the UK. For example, if your initial departure is Lagos and you have transferred in Brussels before the UK, list “Brussels” not Lagos. ALL foreign citizens traveling to Mexico MUST fill out an Official Entry Immigration Form (FMM) prior to their arrival to Mexico. To ease the immigration entry.