Looking for cheap travel for the fall

So I'm planning a trip to Europe in October for two weeks (Oct 6-20). I plan to leave on a Monday and return on a Monday. After researching fares and advice, many people advise to go on Tuesdays, Wed, and Saturdays. I've been messing around with my travel dates and I've found that it really doesnt matter what day you leave really, but it does matter what week you leave. Leaving a week earlier in September would up the price by 100$. I've used a lot of different cheap travel websites to look for tickets and the one that has given me the cheapest price by far has been tripadvisor.com. I found a roundtrip ticket from Philadelphia to London Heathrow on October 6 for 634$ compared to the 700$ + tickets i was finding on kayak.com, sidestep.com, expedia.com,cheaptickets.com. Plus it's on British Airways which I've heard some good things about (but never flown on) so hopefully it'll be a good flight.

Sidenote: I'm also a subscriber to Virgin Atlantic which has had some awesome deals in the past to London ( I went last minute to London roundtrip for 666$ about tow years ago and purchased the ticket the day before). Plus Virgin Atlantic is by far the best airline I have ever been on and I've been on A LOT (i.e. Qantas, American Airlines, American Eagle, United Airways, US Airways, JetBlue, VirginBlue, Jetstar, Airtran, Continental, and probably some others that I can't think of).