If you are looking to go to London, now is the time to go for cheap fares! Delta Airlines has flights priced as low as $245 roundtrip + taxes (add another $150 approximately depending on your departure city and # of layovers). I checked Boston, NYC, and Chicago and was able to get this price through mid-April. The last week or two of April, fares started to rise slightly, but still very cheap for London! Also, from LAX, fares are about $376 roundtrip + tax, but they do have longer layovers in Atlanta.
Remember, Delta allows you to book/pay for a reservation and cancel it by 11:59pm EST the following day. I just did this recently for a flight to Puerto Rico and the credit showed up two days later. These fares will not last long – if you are contemplating the travel, just book it and you can always cancel!
Make sure to use the flexible days search option to find best fares as shown below.