Personally I would steer clear of a 757 for noise and dryness reasons, even though PHL is a fairly short hop.

But for another 757 ITA Matrix is showing this route courtesy of AA: LHR-EDI-JFK/JFK-LHR for £320, and at that price a short train journey to PHL would seem to be worth it to me, compared to the fares quoted above.

For an easier link on a better aircraft, look at AA fares to EWR via CLT or ORD, for around £339. EWR is particularly well placed for PHL. I've no doubt I could get it cheaper still via the European mainland.