Visas and Entry: You need a passport to enter the UK. At present, nationals of EU countries, the US, Australia, Canada, New Zealand and South Africa do not need a visa if they plan to stay less than six months. Regulations change, so always check before you go with your local UK embassy or consulate. There are special rules for children under the age of 18 (only if you are from a country requiring a visa) and for taking pets with you. If either of these apply to you, you should carefully follow the detailed instructions from your local consulate's website.
Documents: Take a copy of your passport and keep it in a separate place. This is always useful if you lose your passport and need to get a replacement. Don't forget your driving licence if you plan to hire a car.
Customs: If you plan to go shopping, familiarise yourself with the customs rules for bringing goods back to your home country.