Its certainly a 1W hub, but not an AA hub. I'm guessing that LHR is AA's largest international destination by seat count, passenger count, and possibly even departure total (YYZ might beat it on departure total). I was tallying the numbers an I counted 21 AA non-stops during summer (JFK, CLT, RDU, PHL, MIA, ORD, PHX, DFW, LAX).