The aircraft concerned spent nearly 24 hours at LHR, then took what was presumably a test flight down to Cornwall and back;
N723AN - Aircraft info and flight history - Flightradar24
It was scheduled to operate AA9276 LHR-LAX today departing at 13:00 but this was delayed and it didn't depart until 15:43;
http://www.flightradar24.com/reg/n723an