You're not really going to knock off 8 hours unless you're doing like LA to Moscow non-stop. LA to London is theoretically only 10 hours in a 747. Sure that'd be like 4 hours in the Concorde, except that it didn't have the range for that so you'd have to add at least an hour, probably two to circle around JFK refuel then take off again.
Not just theoretically, LA to London (LAX to Gatwick or Heathrow) is 10:15-10:30 — there are several flights per day. (interestingly it's 11:10-11:30 in the opposite direction because of the wind patterns).
