One of the new features of Google Flights is the Explore Map shows lowest priced tickets over 6-month period for various airports. Icelandair has round trip tickets from San Francisco to London at $299 through Priceline with a 25 hour layover in Reykjavik on the return or $366 through Icelandair with no layover. Booking through Icelandair also allows you to schedule up to a 3 day stopover in Iceland.
These low ticket prices are travel departures in January to early March 2019.
Icelandair San Francisco – Reykjavik KEF – London LGW – KEF (overnight layover) – SFO
Icelandair San Francisco – Reykjavik KEF – London LGW – KEF – SFO