It probably goes without saying that you need a passport to travel to the UK. You don't need a visa.
London has two airports into which planes from the United States arrive. Heathrow is west of London itself, and British Airways dominates there. Gatwick is south of London; airlines such as Continental fly into there. It doesn't really matter which ariline you fly; choose the one whose schedule and fares suit you best.
Airlines: We tend to fly Continental a lot because they fly directly into Birmingham Airport from Newark (we drive from Birmingham to Oundle), but for most people, the London airports are more convenient, because a) you might want to actually tour London and b) mass transportation is much more convenient from London's airports. I have also travelled on British Air and Virgin Atlantic, and find Virgin quite a fun airline with lots of little perks that make the long trip more bearable.
Time Scheduling/Jet-Lag: The trip from the New York metropolitan area to London takes about 7.5 hours; the return is shorter, about 6 hours, because the winds are usually helping to push the plane faster. We find that travelling overnight on the outward bound (take a 7, 8, or 9pm flight, which will get you in around 7,8 or 9am), then trying to stay awake until at least 1pm, take a short nap, make yourself get up by 5pm, then go to bed at a normal hour, and jet-lag is often minimized. That first day, however, is a killer, so be prepared to count it lost, and certainly don't try to do too much. For best results, go on the "Anti-Jet-Lag Diet" three days prior to your departure; it definitely works.
Time Difference between the United States and England is 5 hours ahead from Eastern Time; when it's noon in England, it's 7 am in New York.
Getting from London Heathrow Airport to London: take the new fast rail link train from the airport into London Paddington Station. This takes 15 minutes from Terminals 1, 2, & 3, and 20 minutes from Terminal 4. The train leaves every 15 minutes at 10, 25, 40, and 55 past each hour. Cost is £10 each way; kids under 15 are 50% off, under 5 free. You can buy a ticket at the touch screen machines in the baggage claim areas. You can have your ticket include travel on the London Underground as well, and probably also to Peterborough; check the machine. Tickets are also available at the desks. For more information, see the Heathrow Express Website.
Once you are at Paddington Station, go via Underground to Kings Cross Station (this takes 12 minutes) and catch a train to Peterborough, which takes about 55 minutes by the fast train on the Great North Eastern Railway (GNER); avoid the very slow West Anglia Great Northern line (WAGN). Cost depends upon the time of day; travel after the morning rush hour and the price drops from about £40 roundtrip to £25 roundtrip. If you are taking the train on a weekend, ask if there is an upgrade to first class; sometimes you can get this for only £4 more. Taxis are available at the Peterborough station at cost of about £12. There is also a Hertz car rental office at Peterborough Train Station.
Getting from London Gatwick Airport to London: Gatwick has two terminals, North and South; most from the US fly into South; British Airways and Delta fly into North Terminal. The non-stop Gatwick Express train goes from Gatwick to Victoria Station in London, leaving every 15 minutes during the day and hourly during the night. Cost is £9 per person one way. You can buy tickets at the desk or at the automated machines. For more info, see the Gatwick Express Website.
Once you are at Victoria Station, take the Underground to Kings Cross Station and catch a train to Peterborough, which takes about 55 minutes by the fast train on the Great North Eastern Railway (GNER); avoid the very slow West Anglia Great Northern line (WAGN). Cost depends upon the time of day; travel after the morning rush hour and the price drops from about £40 roundtrip to £25 roundtrip. If you are taking the train on a weekend, ask if there is an upgrade to first class; sometimes you can get this for only £4 more, but the standard class is fine. (Trains are very good, clean and largely on-time in the UK.) Taxis are available at the Peterborough Station to Oundle at cost of about £12. There is also a Hertz car rental office at Peterborough Train Station.
Car Rental at Gatwick Airport includes Budget, Eurodollar, Alamo, Avis, Hertz, Europcar; all have offices in both terminals. There is also a Hertz car rental office at Peterborough Train Station. Driving is on the other side of the road from what it is in the US, with the steering wheel on the right instead of the left. The vast majority of UK cars are stick shift; it's hard to find automatic cars, and you have to ask and pay more for it. Best advice is NOT to drive a car in London. Also, gasoline (called "petrol" in the UK), is very expensive, about six times what it is in the States. Your US driving license is sufficient to rent and drive a car in the UK; you don't need an international driver's license. If you belong to AAA, pick up UK maps at their office; they have them!