Originally Posted by
TheUmanDay
They seemed to Honor my AA Plat. (Sapphire/Ruby Whatever)
If you are going to be flying internationally regularly I'd suggest you spend a little time reading up on the benefits of AA Plat status - and it's equivalent OneWorld recognition. It's certainly more meaningful than "Sapphire/Ruby/Whatever" as you say.
Notably, you have OneWorld sapphire lounge access when traveling internationally on OneWorld airlines (with exceptions for AA flights to Canada, Mexico, Caribbean and Central America). This has nothing to do with it being a codeshare flight or with your Admirals Club passes/membership.
Originally Posted by
TheUmanDay
Does the Flight end in the normal AA Terminal or over at T5? If so, can I use my Admiral's club pass there?
You will arrive at T5. If you have an onward flight from T5 on BA you can use the Galleries lounges in T5, by virtue of your AA Plat status. If LHR is your final destination and you are in coach, there is no lounge to access on arrivals.
Your Admiral's club passes have no use in T5, and if you are flying back out of T3 even though there is a Admiral's Club there, you have lounge access by via your Plat status so need for your pass there either.