Icelandair has some really good deals from the states, I often see like $199 or $249 from MSP, JFK, BWI, MCO, BOS to cities in Europe and I think they normally allow stopovers in KEF--The fares to KEF aren't that bad either. You can sign up for their webfares at www.icelandair.com

Enjoy the UK while you're there, and plan another trip when you can get a cheapo fare from SFO to an Icelandair gateway city and go from there--I hope to do it someday too.