In spring '09, I did a BA F trip connecting SEA-SFO-LHR-SYD. I'd pick SFO again.

While yes, there can be fog at SFO, there are also enough AS flights that you can build in time for a delay and still make your connections. In a worst case scenario, OAK and SJC are available to reroute if you have built in time to ground transfer, and are rarely affected by weather.

The BA F lounge at SFO, a room off of the main J lounge in the new-ish international terminal, is cozy but pleasant. There's an open no-host bar with the up-level beer/wine/liquor their F lounges get (vs. the J lounge), and cold snacks.

There are two daily 744's SFO-LHR. As of last spring, the earlier of the two got you in in plenty of time to get to the T5 Arrivals Lounge at LHR for a shower, second breakfast/light lunch, and free massage (in a massage chair, with some additional work from a masseuse). The later of the two was pushing it - we bolted off the plane and snagged the last two massage appointments 1/2 hour before closing, and had only about 10 min to shower.

Also, I picked SFO to go on the 744 over the 777. Riding way up in the nose is just cooler IMHO.

There are lots of FT threads discussing F on BA, in general and re: using AS miles. Many people would say to put off your trip until BA is well into, or finishes, the F cabin refit that they rolled out in February. (They may be by the time you are going.) The old F cabins date to the late 1990's, and - while very comfortable - are no longer state of the art. Of course, the service and food is excellent. There are also the fuel surcharges that are now charged on BA award tickets, vs. awards on other carriers.

We went F on BA when we did because AS was ending BA awards to Australia, and, we booked pre-fuel surcharge.