United Airlines puts San Francisco to London nonstop flights on sale for travel from early August through March 2019. Ticket prices range from $502 to low $600s in Christmas week. Travel in August is priced $562 to $580 round trip.
San Jose ticket prices on United are about $10 more for one stop travel via Houston or other airports.
British Airways San Jose to London nonstop flights are priced from $599 in August.
United Airlines San Francisco SFO – London LHR – SFO