You should be able to get some compensation. On a paid F ticket booked several months in advance, I had selected a pair of non-bulkhead seats on a CRJ-700 from ORD to DTW. In fact, the only pair of non-bulkhead F seats on that plane. While we were walking to the gate my seat switched from the aisle in row 2 to the window in row 1. My wife's seat did not change. Neither of the GAs claimed any knowledge of the switch and said they could do nothing. One can never be sure, of course, but the passenger in my old seat was almost certainly not a FAM. (Do FAMs even fly on such flights?). I tried to quiz the passenger about when and how she got the seat assigned but she wouldn't respond or even look up from her phone.
Mine was a far shorter flight than LAX-LHR and I'm not one of these "I have to sit next to my wife" people but, like you, I was annoyed in principle by the randomness of the change and the unwillingness of any agent to explain or resolve the situation. I complained via AA.com and got an apology and 5,000 miles, although I would have been happier with zero miles and an explanation of what really happened.