Virgin Atlantic has published some really unusual 1-day advance purchase fares for travel to London from most of its US gateways. You must leave tomorrow, Sunday May 25, and return in June. If you were shopping around for a last minute fare to anywhere in Europe (you can connect from London on a cheap fare), this is a great bargain. Use a flexible date search on Travelocity to quickly identify these fares, or just enter May 25 outbound and returns Sun, Thu, Fri, or Sat from June 1-22.
Here's an example from Chicago, but we found the same deal from other gateways: