"Premium Service" gets you things like a flat bed and personal IFE. Standard F for international but rather special for domestic F, hence they make a big deal out of it. Offered only on JFK-LAX and sold as P instead of F (costs quite a bit more than F); used to also be on JFK-SFO and BOS-LAX/SFO but I think has been dropped on the other routes, just JFK-LAX is left as P. In any case not relevant to your routing (LAX-LHR), which happens to have the best F service of any route offered by AA -- so you are in for a treat, the best that AA can give you. It is pretty good, sufficiently so that I once switched from BA F LAX-LHR to AA and did not regret it (BA is still better but not by much and AA had a more convenient schedule).