TAP Portugal has SFO to London Heathrow LHR or Gatwick LGW ticket prices at $126.20 one way for travel on several dates from mid-October to early December. Lots of itineraries allow overnight stopover, up to 22 hours. Ticket prices are $350 to $380 with a variety of airlines including United to fly round trip SFO to London. The deal here is best for one way travel to London or if you want to skip out in Lisbon on the way over. And be sure to travel light with only carry-on.
100 TAP Portugal points reduces the price $12. I have 200 points in my account from a sign up bonus last year.
TAP Portugal San Francisco SFO – Lisbon LIS – London Heathrow
TAP Portugal has many more itinerary options at low fares.