I subscribe to the Airfarewatchdog e-mails and occasionally, they come through with super cheap last minute alerts. Like $535 NYC-LON at the end of April and $40 (round trip, with tax) NYC-SQR (where my grandparents live, so I have an alert set up) a couple of weeks ago. Worth an extra e-mail, if you're adventurous.
Also, if you want to travel next week, look into Iceland Express. They're basically EasyJet but they're starting a Newark-Gatwick service via Iceland cheap for the summer. My bf is flying over next week for £267, round trip, with tax. If you don't live in the NYC area, you can always use a domestic airline to connect and still probably come in well under what a flight would normally cost you.