SFO-LHR-SFO has gone to 1xday 747 for a few years now in winter. I thought the 767 was a UA bird, wouldn't be practical to fly a CO bird into LHR T1 given the forced physical separation of UA ops at LHR until T1/2 completion.

As for seasonal demand, the 747s into LHR are adequate in winter, I've had no issues scoring even award seats on those flights in the past, and as I was scoping possible trips in late Dec only last week there were certainly a few options available both in C and F, unlike SFO-FRA (assume this is because easy connections from LHR are basically non-existent now BMI is gone, so it's mostly O/D traffic).