I tried unsuccessfully to use these vouchers on UA954 SFO-LHR on Sunday. I checked in 6 hours early (I was connecting into SFO), am *Gold with LH and was 16th on the upgrade list at the gate. I fell 3 or 4 places short of getting the upgrade (but had a seat in Row 17, the front row of economy plus, which worked out pretty well).

My general impression is that the earlier flight SFO-LHR is less popular, along with the later flight out of LHR. I'd say you have a reasonable chance as *Gold, especially if you travel midweek, but that you shouldn't bank on it.