The Icelandic low fares airline WowAir are offering 50% off all fares with flights booked on the 23rd December ONLY with discount code "WOWXMAS16" entered into the booking engine page, for travel January to 5th April 2017.
Found a sample fare London Gatwick to San Francisco (via Keflavik) for £223.50 return 27th Feb to 9th March, but many other dates available too!
Make sure you tick the "Flexible Date" box, to allow to find cheapest fare.
As WowAir are an Ultra Low Cost Carrier, they charge for larger hand luggage, seat reservations, food, and hold luggage. So you should factor these prices in to the above price.
Flights require a change of aircraft in Iceland which adds time to journey.