The 77W / Boeing 777-323ER is a new aircraft to American Airlines, and even newer on the LHR run, so I'd recommend checking out the existing thread and blogs that are mentioned therein.

Advanced search, search thread titles only, results in threads (not posts) for "77W" (enclose 77W in quotation marks to assure search can find the term, as extremely common terms and those with three or fewer characters will not otherwise be found; letter case is immaterial, as search renders it all into lower case. This will reveal threads about the seats, inaugural flights, etc. etc. (Using the Search Feature on FlyerTalk: The Definitive Thread)

Here's a recent thread in Trip Reports Forum: DFW to London on AA and Return on AA 77W. Post #8 is about the 77W trip LHR-DFW.