Fuh-inally, a decent fare to London! Fly from Philly to Heathrow for $553 round-trip, nonstop, on Virgin Atlantic. We spotted this fare for travel on Mondays/Tuesdays/Wednesdays, on a handful of dates from January through March. Tickets require a Saturday night stay.
You'll have an easier time finding this fare via Orbitz or Virgin Atlantic rather than Hotwire, which may require a little extra clicking around on the calendar before the lower fare magically reveals itself.
For booking info, see our Fare Details.