Somehow I missed the London part during the thread merge. The point about departure taxes is a very important one - that's a pretty hefty tax in a premium cabin out of LHR.

I'm going to guess that SFO-LHR in a premium cabin is going to be tight. You might still want to employ the Star Alliance hub (e.g., FRA) method, depending on what's most important to you. Total door-to-door travel time, best J/F products, exact travel dates, or even a willingness to fly Y if necessary to get the nonstop...all of those factor in to the decision.

But no matter how you do it, if London and Paris are the two cities on this itinerary, guv1976 has it spot on: fly into London, take a train to Paris, and fly out from there. (Again, via a *A hub if you're looking for the better J/F services.)