I agree that row 41 wouldn't be my first choice. However, since LHR-LAX is a long flight, I'd focus less on a specific row and more on the area around me. I'll take a seat in an empty 41 area over a fully booked mini-cabin any day.

The aisle/window pairs for row 31 have significant leg room. However, they are reserved for AA elites and I'm not sure how easy it will be for you to grab one of those. They are usually the first to go.