You're looking to make an open jaw booking, aka a multi city booking. US Airways do operate a nonstop service from MAN to PHL, but keep in mind that schedules are always subject to change.
If your preference is another airline not US then your flight from MAN to PHL will be one via LHR.
Should you book an open jaw ticket with a connection at LHR, please ensure you allow sufficient time for that connection, especially if you will have to change terminal buildings.
"Open Jaw" tickets are priced as round trips. They may a little more expensive than flying a straight round trip, but you don't have to book them as two one ways (which can be very expensive).
TravellerPlus' advice is good. US flies non stop to/from both London and Manchester, and you can get open jaws on BA via Heathrow (the return being MAN-LHR-PHL). Delta/Air France/KLM may offer you one stops both ways. Another option (it may be cheaper) is to look at flying open jaw from Newark with United (non stops to both LHR and MAN), and use the train from Philadelphia to/from Newark Liberty. Obviously you'd have to factor in the cost of the train.Edited: 09 April 2013, 09:04
Play with the multi-city option on kayak.com
It will tell you your options for when you want to travel. If, as is likely, multiple flights are required on travel from Manchester then do check other northern airports as it's possible they might work out cheaper especially if you have to travel to Manchester for the flight home.
Thanks for everyone's suggestions - when did flights from PHL to LHR get so expensive? I was just checking out flights for June and the return fare is over $1,500. I fly to London regularly as I have family there, and I don't ever remember it being this high both on US Air or BA. In fact I flew around the Olympics last summer and even that wasn't so bad.
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