If your destination is LHR, then no, you cannot access any of the airside lounges. Your choices are either connections (where you'll clear security and show your onward boarding pass) or arrivals, where you'll pass through customs and immigration. If you are connecting, then yes you can use the lounge.

AA has an arrivals lounge in T3 which you may have access to, see this thread:
https://www.flyertalk.com/forum/amer...er-thread.html