I cannot answer this as you have not said what country you are from. It depends on that. Assuming you are a US citizen, you don't need a visa, just a passport. It MUST be a passport book.
Also check that any health insurance you have will cover you abroad, or buy travel insurance for the week (this doesn't cost much). This is not required, but if you have an accident while you are away, you'll be glad of it! Emergency treatment under the British NHS is free if you live there, but a visitor like you will get billed.
You need a passport. If you are asking about a US passport then you apply at the post office:
http://travel.state.gov/content/passport...
Entry requirements vary according to nationality, which you don't say:
http://www.skyteam.com/en/Airports-Servi...
If you are a US citizen then you can visit the UK as a tourist without a visa. You do need a round trip ticket.
Check with your health insurance to find out if it covers you in Europe and pays for medical evacuation to home (should that be necessary). If yes then take proof of coverage with you. If no or not sure then get trip medical insurance, which is cheap and sold by airlines & travel agents. Having travel medical insurance is a requirement to enter some countries (ex. Lithuania & Slovakia). Even if it's not, PLEASE do not skimp on this as an otherwise-silly accident could turn into a crisis if you don't have medical insurance.
http://travel.state.gov/travel/cis_pa_tw...
Depends very much where you're coming from. If you require a visa it doesn't matter whether you're coming for a week or a month you'll still require a visa. I suspect you're American, you won't rquier a visa but will requtire a passport.
You'll need a passport to travel internationally.
Your requirements for a visa depend on your citizenship.
You'll need a passport. I have no idea whether you need a visa or not as you haven't bothered to tell us what citizenship you hold and your reason for going to the UK.
That will depend on your country
