As mentioned by channa (and others) you can easily get to BCN for almost nothing. I was specifically checking fares/upgrades post Xmas and found some good stuff.
SFO-LAX-LHR-BCN is available o/w for $245 - with C inventory LAX-LHR - for the entire week starting 26DEC. In fact, four consecutive days are showing C7, the max that EF will display. (Take the kids! Hell, take the neighbors!)
And, while I would never condone any hidden-city shenanigans, some intrepid FTers might just forget to take that last LHR-BCN segment, ending up with a pretty cheap ticket to London. Or, just book SFO-LHR directly for $463.
As for getting home, I don't see anything cheap at the moment, but if you can route via LHR-SEA, you'll find C inventory on 04/05/07JAN.
With Lisbon being even lower...
fyi: re getting home - these instruments are only valid until 12/31.