Virgin Atlantic has San Francisco SFO to Malta MLA on sale for $468 round trip from September 2018 to April 2019, except for Nov 8-Jan 7. The low $468 ticket price is available on many dates. Google flights is useful for finding dates, but no deep link booking to Virgin Atlantic. These tickets are easily found on Expedia.
Virgin Atlantic-Air Malta
San Francisco SFO – London LHR – Malta MLA – London LHR – SFO