I highly reccomend either United Airlines or British Airways, if you are travelling to London. Having flown with both of these airlines, their levels of service (in Economy) is excellent. My BA
flights were a bit more recent than the UA
one, so I may be comparing apples to oranges. BA
has an excellent World Traveller seat with comfortable headrests, lumbar support, and 12-channel video PTV's, accompanied by 12 channels of audio, as well as AirShow. Meal services were delicious, and the Raid the Larder service in the galley between meals was an excellent touch as well.
United as well have PTV's in their 777's, as well as winged headrests. The meal service is good, and the staffing nowadays doesn't include as many "senior f/a's" as they used to. It's all up to timing, depending on what you want. If you feel like connecting, SFO
has a noon-ish United departure which gets you into LHR
first thing in the morning (like 6am), whilst the rest of the flights get you in later in the morning (starting about 11am)
Head in the clouds... yet feet planted firmly on the ground.