This was typed before the last two replies, but failed to submit properly. Some of it is repetition but some of it is still worth saying, I think:

Why aren't you booking this as one ticket? That would likely be a lot cheaper. Is the SFO/LHR portion already booked? For random dates, the cheapest return between SFO and BCN seems to be UA/LH through MUC or FRA. You could try building an openjaw with UA/LH that went: SFO/FRA/BCN**LCY/FRA/SFO and slot in a lowcost BCN-LON where the asterisks are. This will admittedly give you one extra segment, but you'll avoid LHR completely and thus the taxes that go with it.

ETA: look at KLM. For random dates in February (probably not when you want to go) the open jaw into BCN and out of LCY is pricing at $592.20...