I am a US citizen. I have an 8-hour layover in London. Do I need a visa to enter London?
As a US citizen, you're allowed to enter the UK visa-free and stay for up to 6 months, subject to an immigration examination.
So yes, you can visit London
Crazydre is correct.
London has several airports.
You don't say which airport you're arriving at, but most US flights arrive at either Heathrow or Gatwick
Heathrow is West of London, travel times vary from 20 minutes (Heathrow Express) to 1+ hour (Underground, bus/taxi). The Heathrow Express arrives at Paddington which will require further transportation to the tourist sights.
Gatwick is South of London the quickest way from there to London is "Gatwick Express", which arrives at Victoria station. This is relatively close to the tourist sights, possibly even walking distance from Buckingham Palace.
Stansted is North East of London, the express train service takes 45 minutes and arrives at Liverpool Street. This is in the City (East) and is probably another 30 minutes by tube from tourist sights
Luton - not aware of the transport options since I've never flown from it.
City is also in the East and is served by a flight direct from New York on a business class only jet, BA001 & BA002. There is no express rail service, but it is on the Docklands Light Railway which connects to the City of London (Tower Gateway).
Please consider this though:
- You'll need an hour to pass through UK immigration
- You'll need around an hour to travel to central London to see the sights you want to see
- You'll need to be back at the airport 2 to 3 hours before your flight departs, so you'll need to back on your transportation about 3 to 4 hours before your flight departs.
Given this, you might only spend 2 hours in Central London.
Windsor Castle is very close to Heathrow, might be worth going there instead of Central London.