Certainly possible, although you might have left it a little late. Best availability is generally LH via FRA, although you might find 4 seats on UA from LHR. SFO is pretty high demand so you may find yourself having to go via another USA gateway city and connecting onwards with UA or US in F (should still be charged as business). For those of us 'Norn Iron' based, it's very easy to start in Dublin to avoid £80 UK air passenger duty.