I'm sure UA/AF have a full interline baggage agreement. You will need your AF ticket info/# when you check in w/UA @ SFO. I'd allow no less than 2 hours for your connection. 2:30 you should be fine. The flight connections centre if you are flying on UA 930 is a ZOO at that hour, and invariably, as you are approaching LHR, you will go into a holding pattern just when you thought you were going to arrive early. YDMV. If for any reason they won't check your stuff all the way through, you should be fine with a 2:30 to 3:00 connection - in fact, going through immigration and walking to the other terminal can be faster than the FCC and the bus transfer. (This is not taking check-in at the other airline into account).