I recently flew UA931 (14th Mar) from LHR/SFO and used the UK Standby-Upgrade cert without any problems. Was booked in E+, just presented the voucher at check-in - agent confirmed that there were plenty of spaces, then when I got the gate, got handed my upgrade to business. I am a 1P but I am not sure how that may have affected things.

The UA 954 SFO/LHR is usually full in business on Friday nights, so don't hold your breath if your coming back on that one.