For September travel, fly from San Francisco to London for $366 roundtrip on Icelandair.
Searching Orbitz, we found seats departing SFO on Monday, September 24, returning from LHR on Tuesday, October 2.
This is Icelandair's Economy Light fare, which includes a personal item and carry on bag. Standard Economy for these dates is $466 and includes checked baggage plus seat selection.
You'll find similar fares for late September/early October.
Visit our San Francisco SFO fare listings for a complete look at current finds.
Need a room in London? Here's what's available for the above dates on Hotelwatchdog.