Originally Posted by
aamilesslave
Yes, AA should be able to check you in at LHR for your JFK-LAX flight and tag your bags to LAX if necessary (though I'm sure you know you still have to claim and recheck them at JFK).
I don't know if this is a YMMV or I just got a bad agent last time, but I flew LHR-JFK-SAN last year on separate itineraries, and the agent checked my bags through to SAN but didn't give me the JFK-SAN boarding pass. I had to check in again in JFK (and it was checking in, not just reprinting boarding pass - I tried that first). I had 3-4 hours in JFK so I didn't really care so I didn't bother looking into it.