Try this site:
I was on their email newsleter list. Received an email advising that there was some kind of a glitch on Travelocity. Was able to get roundtrip air to London from JFK for $408pp in August. Booked immediately (Virgin Atlantic). The next day the far was gone ....It was very strange. I could not find that cheap fare on any other site - including Virgin Atlantic!
Give it a try, you might save some money - I cetainly did!