American citizens travelling to the UK for a vacation don't need to get a visa but will need a valid passport. During your flight, you'll be asked to fill out a landing card where you’ll have to provide your name, date of birth, gender, nationality, occupation, and the address where you'll be staying in London which must be shown with your passport at the security control.
Australian citizens travelling to Great Britain as tourists for a period of up to six months do not require a visa. If you plan on spending over six months in the country, you'll have to pay a £200 per annum Immigration Health Surcharge from the 6th of April 2016 onwards. To enter the country, you'll need to show a valid passport. You'll also have to fill out a landing card once on the plane providing your name, date of birth, gender, nationality, occupation, and the address where you'll be staying in London.
EU, EEA and Swiss Citizens
As citizens of the European Union and European Economic Area, you won't need a visa to enter the country. However, depending on your nationality or the duration or purpose of your trip, you may need a visa if you're planning on staying for some time.
Tourists wishing to travel to London may need specific documentation depending on their respective nationalities. You can consult the information regarding visas to enter London at this link:
If you have any queries about border control, it's best to contact the British Embassy in your country or consult the information on the website of the UK border control agency:
For more information, visit the official UK foreign office website.