Welcome to FT. Yes since you're flying United you will be able to access the First lounge at both Heathrow and SFO (a partner lounge would not let you in at SFO because you're arriving in F, not departing..but since you're on UA the whole way they'll let you in the first lounge). With 6 hours it will be worth it to stay the international terminal where the first lounge is. Then you'll walk back to the domestic terminal for your flight to SEA. You don't need to re-clear security to move between the terminals, but you will clear security to get airside after going through customs. Your first class ticket should also get you an invitation to use the Arrivals lounge at SFO which is outside security. There are showers and snacks in there..but since you will have access to the First lounge you could just skip the arrivals lounge, go back through security and wait there. The First lounge also has showers. You have plenty of time so if you like lounge hopping try out both. You would also have access to the regular United Clubs in both the domestic and international terminals..but the F lounge is much nicer.