I just cancelled a BA redemption for next month's trip, as I was able to get a M12W fare LHR-SFO (UA metal), then SFO-EWR and EWR-LHR. This is all United Business over and then Continental First, and BusinessFirst on the morning 757 flight back to London, with Z class listed as confirmed at time of booking and M12W fare charged.

Go for it if you can get it, as it's a very good deal.