In PHL, prior to your PHL-LGW flight, I would definately use the Envoy lounge in terminal A-West. It is on the 4th floor, above gate 16. To find it, just look for the elevator and escalator near gate 16. There is also a sign that reads something like "US Airways Club and Envoy Lounge."

You should be allowed to use the Envoy Lounge, as US allows *G passengers travelling on a transatlantic flight to access the Lounge. Inside you will find hor d'oeuvres and an open bar.

Upon you return, it appears that you are flying PHL-LAX-SYD all on the same day. If this is correct, than you are entitled to lounge access in PHL, as you are travelling on a same day international flight. I would suggest that you visit the US Airways Club in PHL, prior to your LAX flight. The club is located between concourses B and C, about a 7 minute walk from the UA gates. To get to the club, after clearing security in the D concourse (where UA gates and checkin are located), make a right immediately after the security checkpoint. There will be overhead signs that say "Concourses A B C." Walk down the hallway (or ride the moving sidewalk) and you will come to the shopping mall. Look for the moving sidewalks and a sign reading "Express Walkway," they are both to the left of the mall, against the windows. Get on the moving sidewalk, and where the first one ends, turn to the left and go up the escalator.

UA also has a club in PHL, but it is truly nothing special, as it is no larger than the size of an average person's living room, with seating for no more than about 15 people.

A map of the PHL airport can be found here http://www.usairways.com/travel/maps/terminal/phl.htm