Your MCO-PHL leg could arrive at concourses A, B, or C, and the flight to LHR will leave from A (I believe BA has a gate that they mostly use from PHL). 50 minutes should be sufficient, so long as the incoming flight is on-time. All of the gates at PHL are connected after security. So long as no one in your party has problems walking, it shouldn't take more than 15 minutes to walk between the furthest gates, between A and C.

There's a thread on PHL over in the AA forum, with terminal maps, and other helpful information in the Wikipost at the top. Please take a look and feel free to post there if you have further questions:
AA Guide: PHL / Philadelphia International Airport - MCT, Connection, MCT, etc.