BA only launched LGW to Oakland to compete directly with Norwegian and there's absolutely no need for them to start a route to San Francisco as the destination is well covered by the airline from LHR.
With regards to FLL, personally when ever I have been in Miami I've always thought I wish that I was in Ft Lauderdale, also the opposite when I'm in Ft Lauderdale I wish I was in Miami even though they are only 30 miles apart I never actually visited both together but since BA started FLL I can fly to one and out the other as both Ft Lauderdal and Miami are great destinations to visit and having flights from both airports makes it a lot easier to visit both, plus the fact that the company that I work for has business links with the two of them!