MIA-LON is four flights a day. One AA 777-200 to Heathrow, one VS 747-400 to Gatwick and two BA 747-400s* to Heathrow, I believe UA also operates one-stop service via IAD during the winter. bmi has applied for MIA-LGW and MIA-LHR services (and granted MIA-LGW, I believe). How far they will take that, I do not know. I would not be surprised to see either a third daily BA flight or a UA flight to LHR. I think SFO is 6 or 7 flights a day with United, Virgin, and British Airways, but I'm not sure. After NYC-LON, I believe LAX-LON is the next biggest market, followed most probably by San Fran-LON, Chicago-LON, Washington-LON, and Miami-LON.
*The flight is currently using two 747-200s because the two 747-400s used for the route are undergoing conversion to BA's new four-class layout, which will arrive in Miami sometime in early 2002.