Do you have any Virgin Points? You can often get decent value on VS points-wise to BOS. This could combine well with SFO-LHR on BA.
Not quite your dates but you can get LHR-BOS in Upper Class for 29,000 points + £497 "tax" on 8th November, currently in their best Upper seat on the A330-900 (aircraft subject to change!)
On 9th November you can get LHR-BOS for 17,500 points + £430 "tax" in Premium Economy or 6000 points + £215 in economy. On 10th November 10,500 points + £370 in Premium.
VS are not great value for SFO for your dates.