This is one of the last international flights out of PHL -- the PHL-MAN and PHL-EDI ... so wouldn't be too confident about misconnects. And, it is even in the back.
Also, going through LHR really isn't a good option although the AA/BA 747 code share has two J seats available. Looks like all the morning flights from LHR to Edinburgh are sold out.
Assuming no misconnects, PHL-MAN would be the best option.
Curious -- does AA have the ability to provide a train voucher?