The US ones are in F, but all on 2-class planes. Good for FF credit I guess, but at the cost of 150 miles I would go LHR-LAX-MIA with LAX-MIA on AA1520 which is F in a 3-class 777 and somewhat nicer.

Otherwise I can't see a way of doing much better and staying in F. There are some options with a few more miles if you go to places like SJU. That books as D although the seat and the service are the same as domestic A. I don't know the QFF scheme so not sure what the impact would be.