The below on VS is £3359 (if booked through me with fees)
VS 3012 20DEC EDILHR 0640 0810 OPERATED BY AER LINGUS FOR VIRGIN A
VS 19 20DEC LHRSFO 1100 1415
VS 8 04JAN LAXLHR 1855 #1335
VS 3007 05JAN LHREDI 1515 1645 OPERATED BY AER LINGUS FOR VIRGIN A
United are £2755 for the 20th to the 3rd through New York but this does mean the long internal connection, though currently the flight from EWR to SFO & LA to EWR is also on a 757 with flat bed Business Class (but these could easily be plane changed).
Air France through CDG is £2843 from 20th to 3rd – return from LA is A380 but outbound to SFO is 772. Routing through AMS looks like old plans and products on KLM.
Might be able to find fares slightly cheaper online.
These look to be best routing options unless you want to use BMI regional for example and overnight in Zurich before using Swiss for example which I guess isn’t desirable.
Hope that helps and gives a guide to what is possible.