You should be fine. If your flight from London arrives late and you end up with less than two hours to connect, an agent will meet you on the jet bridge and provide you with an orange Express Connection pass, which should speed you through immigration, customs and TSA security. The only thing that can't be expedited is your checked bags to the luggage carousels. Should you miss your connection, American has nine daily non-stop flights from JFK to LAX (and an additional four code shared JetBlue flights), so you should be protected on a later flight.