One more solution from Europe to USA based on a Star Alliance promo sale (see also our solution from Scandinavia / France to Denver from €279) this time discounted deal from Italy (Milan, Venice) or the UK (London) to California. Return flights San Francisco are available already for €327 or from London £281. Base fare excludes hold bag otherwise it is for a round trip flight incl. all taxes and fees. Again you can fly on various airlines such as Lufthansa, Swiss, United or Air Canada so choose the routing and carrier you prefer..
Travel restrictions of this Lufthansa (and their partners)promotion: Fly from 7th of September to 18th of December 2018 or 10th of January to 4th of April 2019. Max stay 3 months. Promo sale is running till 31st of September or until the cheapest seats last. Need an accommodation in San Francisco or Los Angeles you can use service of Hotels.com or Booking.com. You can compare all travel dates directly at website of the airlines.
London – San Francisco from £294 (Non-stop flight or from £281 with a layover – outbound only)
Booking sample of promotion offering Star Alliance cheap flights to California from Italy €319 or the UK £281! Your outbound flight from London from San Francisco would be on Monday, 28th of January and returning back from USA to the United Kingdom on 8th of February 2019.